I can really relate to your feelings and situation. I myself am in a relationship of twelve and a half years, and am soon to be married (in August). However, life right now isn't great, for the past few years it's been getting gradually worse in fact. Our sex life is near non-existent, neither of us have many friends or much of a social life, she is constantly tired, stressed and pressured from work and I am trapped in a dead-end minimum wage job that I hate.
Despite all that I love the missus very, very much and, like you, am not prepared to throw our relationship away. I tend to go through little cycles of self-doubt and self-hate mixed with anger, depression, frustration and general grumpiness. First I blame her for not wanting sex, then I get sad and feel sorry for myself, even entertain the idea of cheating. Then I feel extreme guilt, more depression, then I realise I am probably more to blame than her. I come to the revelation that I have been far from the best partner, and that I could and should be doing much more for the relationship.
She wears the trousers you see. She is the bread-winner, she makes all the decisions, she is the organised one who decides what we have for tea, makes sure we have enough toilet paper and that the bills get paid and the joint-account is topped up and the cats have food. I'm the one always forgetting that his car tax is due, where his wallet is, who's birthday cards need posting, etc. The chores that I am responsible for I let fall behind. We've both let ourselves go and have become complacent with each other. I take her for granted.
I've vowed to turn this around, though. We're on a pre-wedding diet and I joined the gym last week. Starting this week (we're both off work), I'm doing most of the housework, all the chores will get done, I'm going to start being more decisive and self-sufficient. I've told her she's to relax and not think about work. The goal is self improvement and to start picking up the slack, taking the strain off her. Hopefully, this will result in improvements in my mood and attitude, likewise helping her to relax more and in turn feel happier. This will make me happier. And hopefully this cycle will reinforce and perpetuate itself.
So, what was the point in that little life story? Well, I see my own troubles in your post, and I just wanted to explain that you're not the only one, and that this is how I plan to turn things around. As for you feeling you can't give her what she had from younger ex-partners, well... you're the one who has been married to her for the past two decades. So, either whatever it was she didn't really want or need, or whatever it is you do give her is better anyway.
You can always go for the gold-standard in advice given by this fantastic community: communication. Talk to her about your concerns and feelings. If talking is too hard try an email or a letter. If all else fails, you can always come onto places like this and vent, and you'll find others who will listen, and try to help. At the end of the day, when you feel like this, changes need to be made. And big or small, they usually need to start with yourself. At least, that's the conclusion I've come to.
Hope this helped in any way. Take care and good luck.