How do I create a boundary with an ex-boyfriend, and is it possible to be friends when it was not a mutual decision to split?

I ended a relationship with my boyfriend in October. He was not happy in the relationship, but he didn’t want it to end. We had been together for 3 years and lived together for 18 months.

We’re trying to remain friends, but he keeps bringing up our relationship (where we went wrong, what we could’ve done better, etc), my previous marriage/past that happened long before I knew him. I don’t think anything prior to knowing him is his business (early in our relationship, I shared personal info because he felt like a safe place), and I’ve told him I will not discuss it anymore (I haven’t always upheld that boundary OR I get mad that he has asked again).

He also gets cranky when I don’t respond to his texts immediately, and he will DM me on Instagram and/or Facebook, and then if I don’t respond, he’ll call me, and usually we get in an argument. This usually happens on the weekend so I feel he’s concerned I’m seeing someone else (he’s asked me on multiple occasions), and he needs me to respond so he can put his mind at ease.

He is friends with many of my relatives on Facebook (some he’s only met once and I’m not even close to), and he comments on their posts. I just posted 20 photos from Xmas, and he liked each individual photo, and he does the same on my family members’ posts, commenting & liking everything. I know he can do whatever he wants, but I feel he’s doing it to keep himself relevant and/or to get my attention. I don’t want everyone to unfriend him because he’s going through his own struggles and will likely feel hurt, but it bugs me.

We had an argument the day after Christmas, and I told him I would not answer any further questions regarding my past, and that our relationship is over for good. He seemed to understand. We’ve had a few short “how’s it going” texts since then, but he asked me the other day if we could talk on the phone. I told him yes, but I’m purposely not calling him and dreading a call from him, partly because I don’t know if I’ll hold my ground AND not allow myself to get angry.

Any help is appreciated. Thank you!