AITA I was speaking to someone else while in a relationship

A few years ago I (28f) was briefly seeing this man (32m) but during and after a few months his mental health was so bad it wasn’t a good time to be in a relationship nor continue sleeping together but we continued speaking as friends. We had both suffered abusive relationships in the past and I also had suffered bad mental health so I related to him and we had helped each other through bad times. Our communication was always sparse and at some point we both found significant others and he would message me every few months just to catch-up and see how each other were doing. Nothing more to it. We spoke about our partners, new jobs and families and then he’d reach out again a few months later. That lasted just under a year and our last conversation was in March 2024.

Me and my current partner (29m) have been together 2 years now and we were having a few drinks in the pub and the conversation of ex’s and wild stories came up, for some reason this guy came up in conversation and I had it in my head I had told him about us messaging me but he doesn’t remember it and it was such a long time ago and such brief conversations I possibly didn’t tell him- despite now knowing I definitely should have. He gathered we had been speaking at the same time as us being together and wanted to see our messages, I showed him and he is understandably very upset. He said he knows I didn’t cheat on him but doesn’t understand why I’d continue speaking to another man I’d slept with while with him and said I’d be upset if he was speaking to another girl he was sleeping with. Despite knowing my intentions were pure and being there for him when this man was trying to kill himself, I still feel guilty and I shouldn’t have spoke to him while in a relationship. Any advice how to fix this?

TLDR: aita for continuing to speak to a man platonically I used to sleep with while in a relationship?