Why would my boyfriend invite his ex to his graduation party (I'm not going btw)?

My boyfriend is about to graduate from college and I just recently found out after helping with invites that his ex is invited. I'm not going to his party because I'm still in highschool (18) and it will just feel awkward. He has his older friends around and they're having fun talking about the future after school and I'm just there. I may come for a bit but then leave. His ex and him have a long history together. They were once engaged and he broke it off claiming she was too harsh with her sarcasm and that he fell for me. Their engagement was in high school and they wanted to get married around 19. I think they were together for 2 years? It was just too rushed and a mess in my opinion.
I'm so insecure about us and this just tops it off. I feel way too young for him. A week ago I was out of town visiting relatives and he went to a friend's party. His ex went there too and his friend told me he saw them outside talking and drinking. They were laughing and acting like good friends and that they were even caught dancing together. His friend told me that they kept giving each other looks throughout the night and he walked her to her car. I talked about this to my boyfriend and he did not deny anything. He said he loves me and I need to trust him. He even bought me flowers?
I don't know what to think of this. My mind is filling up with nasty thoughts about them. He knows I'm so insecure. And I felt like I was the cause of their failed relationship.
(We have been together for 9 months about)
Why would my boyfriend invite his ex to his graduation party (I'm not going btw)?
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