Welcome. Many step parents that find this site decide that disengaging from parenting their step child is the only way to stay sane. So wanting to stop dong things for your sd is perfectly normal around here and not ridiculous at all. 

Another disengager here - 17 yrs or so since I did it - my SDs are now 23 and 25, never regretted doing so.   Unlike you I never had a good relationship with my SDs - their mother saw to that - but there are many like you on this site that sweated blood for their SKIDs only to have it thrown back in their face at some later date.  You might want to read more in the disengaging section of the forum here. 

She's 16? There's somewhat of a light at the end of the tunnel if you and your husband plan properly. I would devote my talks with DH to making launch plans for her future.

Welcome to this site, it is a great place to vent. I came here for the same reasons.  I gave everything I had to give to my SKs and treated them like my own. It truly has been the most thankless experience of my life. I finally threw my hands up in the air especially with OSD and between completely disengaging and therapy with SO just try to manage getting through the next few years. 

Welcome; I would highly recommend you reflect on what you want from this relationship, does it align with what you have and if it doesn't is there a way to get where you want to be. A lot of posters will suggest you leave before you have a chlid to keep you attached to this situtation. These suggestions come from a good place as most of them come from people who have lived through it. Remeber you are not likely to be the fairy tale exception to the standard and really considere what they say to you.

Secondly, there are plenty of "teen" based posts here with a lot of comments and advice by  members like myself who have been here for a long time. Some people will give you the advice in a harsh way so if you read through the comments you can prepare for what might get thrown at you. I will be the first to say it, since I know its coming--You have a DH problem. Most tyranical stepchildren are a result of one of the following; a Disney parent (the fun-only parent), an aggressive parent (competitive with the ex), or one who ignores all problems because they think their child is the secondcoming OR too overwhelmed with their troublesome child to actually deal with their behavior. Identify what you are dealing with, look up suggestions in the comments around here that have worked for posters and see if you can use any of them.