Wagon Crash

Sex should never feel like a duty.
There are so many things that can effect sex.
Sometimes we are just tired and not in the mood and that's ok.
If you don't want sex because you aren't attracted to her or there is too much baggage with the relationship at this point then that should be addressed and worked through to figure out what you want for yourself. It's fair to say people grow apart in marriage and it happens.
Even me in my thing that I have with my AP, some days he had to cancel at the last minute and I was glad because I was beat down tired and of course I'm comfortable to tell him not tonight I'm tired but I tend to push myself.
A few months ago he texted me and wrote that his schedule was so busy and he couldn't see me that weekend but he wanted me to know that he always wants me. I think in essence that is what it comes down to with sex - the wanting. As long as you want sex with her and it's not a duty then you will be ok but I think in your case there are a lot of issues to work through that are preventing you from truly wanting her and really does she deserve to be wanted?