@LanaLielaLie What makes you happy?
You say you're ashamed of the overlap, so you obviously don't take pleasure in having won the 'pick me' dance. Instead, you feel guilty that another woman got hurt.
You say you're anxious he'll go back to his ex at any moment, so you obviously can't relax in his company, thinking about how things might end.
You explain that he pressures you into doing things sexually that you don't want to do, so you're obviously not happy in bed.
I know you don't want this relationship to end, but everything about it is making you unhappy. What makes you think you would be unhappier on your own?
There is absolutely no shame in realising you've gone down the wrong road and turning that car around. You don't have to continue in this direction.
I sense you think this is all you deserve or all you can get, or even some kind of punishment for being the other woman. I can assure you, none of those things are true.
If a friend was in the same situation as you, you wouldn't judge her, would you? Instead, you'd give her a hug and tell her she deserves more than this. You know this isn't right.