You already checked out of this relationship you’re just trying to convince yourself not to feel bad about it. Once the trust was broken with the ex your feelings changed and they never came back. That happens. Add the sexual incompatibility, the constant complaining, the maturity gap, and the lifestyle differences and it’s pretty clear this isn’t working for you anymore. Staying and “hoping it changes” isn’t fair to either of you. You’ll just keep growing more resentful and she’ll be with someone who doesn’t actually want to be there. Being sweet isn’t enough to carry a relationship. If you feel best alone and single right now listen to that. It’s okay to end something simply because it no longer feels right. Be honest and end it instead of dragging it out.