First post here, so I apologize now for the long vent. 
i have an almost 13-year-old stepson who I have raised for the past 5 years when his dad and I got together. I have 2 bio daughters from a previous marriage, one also almost 13, and the other 7, and then my husband and I completed our family with a daughter of our own this year. 
Back story: my stepson was not planned by my husband and his ex, they forcefully married and then ended up separating when SS was 2 while my husband was in the military. My husband would visit and check up on SS over the years to which the Bio mom (who I like to refer to as egg donor), would reply all is well. Fast forward to SS being 5 years old when my husband's military term was up, and bio mom tells hubby that he needs to take SS, she "can't care for him anymore" (in my and my SIL's opinion, the child support payments were stopping and he was just a paycheck for her). Husband and his family go to pick up SS from her and discover she had lied about many things, and that he had missed a good chunk of kindergarten, she was taking him out to bars to live her party life, and the living conditions were horrid for a toddler. I imagine he was stuck in front of a tv with a bag of chips while she slept off her hangovers, speculating of course. BM has lived 2 hrs away ever since and has MAYBE visited/called SS a total of 6 days in 5 yrs, half of those being that we were in the area and stopped for him to see her. 
From 5 years old to 7 1/2, my husband took on full-time dad and had issues with his son he didn't know how to handle (routine pee and poop accidents where he'd hide his underwear in the closet, behind the toilet, etc.), social awkwardness, poor eating habits with the exception of very few dishes, poor behavior at school, etc. 

Husband and I started dating when SS was 7, things were great, and I invited him and his son, at this time 8, to move into my (I consider our) house and we all movied in together on the day I closed on it (as I was in the process of purchasing after my divorce finalized). For the first year, things were ok, SS still had issues but I never wanted to overstep hubby's parenting of his son or push myself on SS being a stepmom. Then when SS was 9, my husband entered into a demanding, full time night shift position. Things have only gotten worse since as I've had to progressively become the primary parent and disciplinary. 
Now, my stepson's issues have only escalated. His potty accidents never ceased at least until he was 12 years old (his room always stinks but i've given up checking). When he was 10, I got tired of scrubbing his underwear and deep cleaning his bed, and decided it was time for him to wash his own clothes, so back to hiding them it was. Tried giving him kid diapers to which he absolutely refused and gave me arguments for him to wear them. He will not brush his teeth or bathe worth anything. He will lie and run water, and I have to send him back in again after a nasty ass stare down from his devilish eyes. If I tell him "do this, finish your homework, eat that, or choose between these snacks," etc. he will without a doubt be defiant and go out of his way to do the opposite of what was asked of him.
When confronted for doing wrong, he blatantly LIES. But I've tried the other extreme of letting him do what he wants all day and not ask a single thing of him, and he still won't do what he's supposed to do. Once i didn't say a word all day to him, and he literally played video games for 11 hrs until i went over, softly told him that was enough and to go get ready for dinner. He never got dressed, brushed his teeth, or grabbed anything more than crackers all day. He threw his hands up and yelled at me that he "barely was on it!" He will slam things, hit walls, break his toothbrush, destroy anything new for the home I get, write on furniture, throw away anything I bought for him when sent to his room after his behavior (xmas presents, clothes, shoes) stating he doesn't use them or doesn't like them. I really think he is the cause of a recent nail in my tire as I have too much suspecting evidence. 
He has always had disciplinary or behavioral issues at school also. With a written behavioral evaluation from his school and a verbal evaluation from his pediatrician, he was diagnosed as having ADD, ODD and no incontinence, he CHOSE to have accidents. And for the record, before this kid, I NEVER believed in these diagnosis, and very wrongfully chalked it up to lack of parenting. I finally had enough 2 yrs ago and got hubby to agree that SS needed therapy. But with husband's schedule, I had to be the one to take him to therapy every week to no avail.
Finally one morning before therapy, i ignored SS's attitude while telling him to get ready as everything is an argument- eating breakfast, brushing teeth, getting on clean clothes- and i sat down for the HALF HOUR and just wrote all the issues he was given me or my girls (as he particularly likes to pick on my younger daughter, now 7). I got to therapy, told the therapist that i was sitting out as i had my other kids with me this time, gave her the paper and said "here was our morning." The therapist came out after the session, and apparently getting SS's reaction to the issues she now read about, and said "this is way beyond me, he needs to see a psychiatrist. And hide your knives."

We need to bring him, but there's no decent ones around here. I've put so much time, money and energy into this kid, while trying not to let him hinder the time I have with my older daughters on my week (shared custody with decent co-parenting with my ex). My husband is at a loss with what to do, knows his son has issues, admits that I'm a great mom as he claims my girls are perfect, but yet, at times blames my coldness towards his son to be part of the problem. I don't want to be that way, i used to never even curse, but this kid has brought out the absolute worst in me, and i honestly hate him for all he's putting me through and trying to take away from my girls. 
When husband's home, I try to step back and let him deal with him, and he will support me in disciplining for the most part, but also hates to see his son in a constant state of grounding. I cook and clean and have a self-employed business as well while taking care of our baby and my other girls. I don't put much on the kids as far as chores as I want them to be a kid, basically just "clean your room, shower, brush your teeth, eat dinner, feed the pets, do homework and use manners." THIS, he can't do. I'm firm with my consequences if you don't follow rules, so if I say no tv for the weekend, there is no tv for the weekend. No empty threats here. I set the same rules for all the kids, and I don't agree with being easier on him just because he's difficult as this also sets an example for the younger ones. 
Being the main parent while my husband works at night and sleeps during the day sucks with a stepkid like him. I used to call husband at work and tell him how SS was acting, but with husband's line of work, it's not best to keep distracting him and unloading home problems on him while on duty. If I try to ignore SS while home alone with him, he will purposely try to annoy and disrupt me if trying to watch 20 min. of tv, or keep hitting his door if sent to his room, and then if confronted, he acts naive. Same with school phone calls or while talking to his grandparents about his behavior with me, he's always "the victim."

I'm so done, and have seriously contemplated divorce for peace of mind. I do imagine husband will be just as eager for SS to get a life of his own at 18, but I don't know how to deal with 5 more years. I love my husband soo much, but I despise this kid just as equally. I never thought I'd be an evil stepmomster, but I don't know what to do to be happy or to find a middle ground with this kid. Any advice is welcomed!