Delayed anger after discovering my boyfriends infidelity

My boyfriend over a year and 3 months went to a massage parlor to get a happy ending. It was a stressful month for us, and I caught him. He didn’t come clean.

Having been lied to in the past, I’ve always vocalized how important fidelity and trust is, as well as disclosure. I had to know I was safe in the relationship. A week prior, he promised to never do anything to break my trust and then promised if he did he would disclose.

In the wake of discovery I was gentle, kind, supportive, listened, I was there for him as he seemed to be struggling. It felt healthy and we got him help with a therapist. It’s a bigger issue around sex for him.

Now, it’s been 2 weeks and last night I just….slipped into anger. I spoke to him in a way I have never before, bringing up his past, what he’s done, how selfish it was. It was almost like I went into a different state. I’m not an angry person and I don’t deal with having issues controlling my reactions but it just happened. I’m meeting with my therapist today but wondering what other people’s experience has been around thin type of thing?