So, I'm finally back together with my boyfriend-forgiveness?

So my boyfriend of over two years dumped me in February. We got back together this past week. We had been planning a future and everything. We were each others first EVERYTHING. But during those months my 19 year old boyfriend made out with 9 people (3 guys) and slept with 1 girl and also did other stuff with her. My 17 year old self is finding this hard to get over. Really hard to get over. I thought we'd be each others first and last everything and I was wrong. But he's PISSED at me for being sad about it and says is none of my business since we weren't dating when I know if it had of been me with this new experience he'd be torn apart. I don't get mad at him or anything, I just get sad. Has anyone had this happen? Any advice?