Why do I feel insecure that my boyfriend is going to leave me for this other girl?

We've had our issues where we were both drunk at a wedding and he was flirting with another girl who is a family friend of his. When I bitterly asked him if he was flirting with her, he got so upset and told me that I ruined the night by being jealous. Because I asked him this, he went with the girl and left me. He told me to go home and that I'm not invited to the after party with him and his friends. So I left crying and driving home alone, and he ended up yelling at me as he left with the girl and his friends.

I've been insecure about this girl for a while because of how he talks to her, but I thought maybe it's all in my head. The day after that incident, he completely apologized to me. He told me that the girl is awesome and he reminds him of her sister which he had a huge crush on when he was a teenager.

Ever since, I've been extremely insecure and decided to talk to other guys to keep my options open. And when someone asked me on a date, I brought it up to him and asked if we were exclusive or if I should proceed to continue dating. He looked like was not okay with me dating other guys, but told me to go and date if that's what I wanted to do. But he tells me he is only seeing me and only wants to be with me.

What are your thoughts?