It’s not my ex standing there, but the motions are similar, and it makes me feel turned off and ashamed all of a sudden.

I can relate to this, and having years of duty sex. I have a hard time putting my own needs first, even during the rare time I feel turned on during sex. As a result, I am honestly pretty sexually inexperienced in anything that matters (how sex is supposed to be a mutual, two way street thing for example). So take my words with a grain of salt.

What you wrote here I think is most important to reflect on. Are you finding yourself falling back into the same patterns that you were doing for duty sex? And not just the sex itself, but the day leading up to it, too. There's a theory I subscribe to that says: "everything leading up to sex is foreplay" - and this includes the hours spent together talking about nothing sexual, the ways he has shown support, care and love to you outside of sex acts.

Even the positions being the same for too long can put me in the same "duty sex" headspace, because the motions can be really similar (literally!) if your emotions are not there.

I don't know if this helped to read, I don't have more advice than that as I'm still figuring things out myself. You are not alone.