My bf is mad that I tried to get closure with my ex

Tldr; My bf is mad at me for texting my ex.

My (20F) ex (21M) left me last winter after 4 years. After a month, he regretted it and wanted me back, but I was onto other things and didn't think about him anymore, so I asked him not to text me. I told him that, after some months, I would text him when I saw things more clearly. Sometime after this, I started meeting a guy (20) from my workplace and we were casual for a few months until we started dating at the beginning of this year.

I feel great with him for soo many reasons, but I couldn't shake the thought that I had promised to text my ex. I didnt want to go back, but I knew that he was struggling mentally (for our breakup but also for unrelated issues), and so last time I was back in my hometown I send him a text clearing things up: forgiving him for his wrongdoings and telling him I was okay and that I had met someone new. I said I didnt expect a response and I didn't get it, which was great. I told my bf when I came back and he got mad at me, because he didnt understand why I did it. I tried explaining that it was just something that I emotionally needed to do to close that thread, but I hadn't gotten to it earlier because I was sorting out this new relationship with him.

Now things are uncomfortable and I don't know what I should do.