My Boyfriend 26m slept with a fwb after three dates but before exclusivity. I 25f don’t know how to feel?

My boyfriend 26M and I 25F have been together for 6 months now. Last week something came up an I asked the last time he slept with someone before me. He couldn’t really remember when it was but according to his description it would have been 3 weeks after we started seeing each other, so 3 dates in. He was honest about it all and told me he didn’t see it as a problem back then or now which is why he never brought it up. He didn’t see us as together so thought it was ok. He did admit he felt a bit shit after it as he knew he had feelings developing for me but didn’t think I’d have a problem with it and it made him realise even more so he wanted things to progress with me. She was a girl he had casually been sleeping with once every two or so weeks and had no feelings for her. During these three weeks he had been taking to her but only slept with her the once. I’ve realised that the night after he slept with her we seen each other an had the exclusivity chat, which is when he cut things off with her.

I just don’t know how to feel, I love him so much and I truely believed he was my forever but now I can’t stop thinking about this. It was early but over those three dates we spent a lot of time together, said we liked each other, spoke about what we’d want for a potential relationship, plus we were messaging everyday. I remember during this time we had asked each other if we were seeing anyone else which we both said no. He’s told me he didn’t mention it as it was early an he didn’t think he needed to as he was planning to end things with her regardless of us. He said he didn’t plan to sleep with her again but apparently she’d been asking to see him alot an he figured there was nothing wrong with seeing her that last time.

We had not had sex during this period or even kissed. But we were getting closer emotionally. I remember telling him it was ok if he was still chatting to other girls but I also said I didn’t want to be under the impression he was only seeing me and not sleeping or talking to anyone else which he responded that he was only interested in me. He’s told me he meant that but didn’t think sleeping with her changed anything towards our progression at that time of things. Since hes told me he understands why I feel hurt and wasn’t the right decision but doesn’t think it’s anything to worry about now and doesn’t want to loose me over this. I overthink so much and I just don’t know how I should feel about this