Step dad to an autistic child

I'm a stepdad living with an autistic teenager. I care about my stepchild and my wife, but I'm finding the dynamics of being a step-parent in an autistic household really difficult. I don't want to identify myself or my family, but I'd really like to talk to other stepdads/step-parents who have experienced something similar.

I  have been diagnosed with ADHD and suffer with RSD, and have done all my life. So any sort of behaviour that I perceive as disrespect, it hits me in a hard way. For example, a grunt instead of hello, not saying thank you, not saying goodbye. I have really struggled with the child being home educated for the last 2 years in the sense that I feel she has just been left to do what she wants. My wife hasn't had great support from the government but in the same breath I don't think she's pushed that hard. With my job I get a day off in the week, however I find it extremely difficult to relax knowing someone else is in the house, not doing a lot, all day long. 

It's got to the point where I'm not sure if I can continue my marriage. My wife is an amazing woman, would do anything for me and if it was the other way around, she would support and help me. Which makes me feel so bad. Truth be told, she's never " let me in" in terms of helping and dealing with the autistic child. 

My upbringing was completely different to my wife's. We had very little, I was always taught to always look after our things and help with household chores. Respect and help the people in your house. I work long hours so the time I do get at home, I end up cleaning and trying to maintain it and when my step child is doing nothing but stay in her pyjamas all day and not helping, it infuriates me. 

There's many moving parts to this, but one thing I really struggle with is that I've known my step child for 7 years and have seen the transition from when she was 9 to now and I've seen her learn how to get away with things, not do things. I've seen her lie and manipulate situations. My wife is so scared that her child will do something stupid, she lets things go. But I see it differently and it's frustrating. 

I just wanted to reach out to see if anyone had any advice, have been in this situation etc. I find it incredibly tough, I feel for my wife and step child that we are in the position, and get really frustrated with myself that I can't seem to support my wife when she needs it, due to my upbringing, values and beliefs. I don't have a bond with my step child. I used to, we got on so well. Now it's just dwindled in front of my eyes and I am losing the desire to even try and get it back. 

I know I'm opening myself up for criticism with this, however I don't think I'm the first stepdad to be in this position, so any help is welcome!