Something doesn't make sense here. You guys have been together since you were about 18, you look better than ever and she's gaining weight but you clearly stated SHE is the one who isn't interested in you. Why do you think that is? Plus she's not taking care of herself ... sounds to me like you're s/o (wife/girlfriend) is short on time or energy, or unhappy about something. Either she's stressed, depressed, unhappy with life in general, or unhappy with you.
I gotta ask, while you're working out, going to clubs and flirting with younger women, while she's doing what? Staying home by herself?