To the 23-year-old bi woman--
I have been in relationships I didn't want to be in and felt similarly irritated and annoyed by the other person. I have also entered into relationships I wasn't really all that into from the start, and that is why I felt similarly to how you describe feeling. Dan is right, you shouldn't have started dating the 40-something if you weren't into him. You enter the relationship thinking you are being nice because it makes THEM happy. But since being with this guy doesn't make you happy, those feelings will show eventually and your unhappiness will make him unhappy. Believe me, I've been through it. So if you aren't into someone but he or she is into you, do what all DARE officers tell you to do and JUST SAY NO. In the long term, it's the nicer thing to do. You won't feel repulsed by the attention of someone you are in a serious relationship with if you sincerely care about him or her.

Short of fear that the dude might act out and get hostile, I don't know if I agree about lying totally to the guy that's overbearing and doesn't realize you're not attracted to him. It's gonna be incredibly painful to hear, but maybe some diluted version of 'I don't find you physically attractive' would help. Even if it was some variation of 'We're not sexually compatible.'

White lies are permissible sometimes (the shorter the relationship, the more they are, in my opinion), but other times you're enabling that person to just continue behaving the way they always do, being as clueless and as emotionally disconnected as they always are. Getting hit by a truck (that's full of truth) can be the thing that some people need to really look at themselves.

yep I agree with both prior commentors. Staying with someone to make them happy when you are not, will more than likely fuel fights, and be emotionally draining. It will certainly tear the relationship apart in a horribly unkind way (probably from both sides). And leaving the 40 something clueless is also bad news.

Hard to think of exactly the right thing. Maybe breaking up with him with an easy white lie such as "I'm just not really ready for a commitment." AND "Let's stay friends" AND actually for a bit go out as friends. Keep in mind to wait a while so he doesn't feel like maybe you will get back together. Go out have a coffee or something, and politely mention "some things" you have been thinking about your prior (but absolutely over) relationship, and maybe he'll learn for his next relationship.

Also if you are thinking of going back to the old fling, maybe you should start friends first and talk about what you were unhappy with in HIS relationship to you.