My (19/f) bf (20/m) started talking to a girl soon after we stopped talking when we broke up. How do I get over this?

I know he didn’t do anything wrong, I never hold it against him. He’s a human being and allowed to explore things with other people. We broke up for awhile and in the middle of breaking up we had a month of talking romantically but then decided it wasn’t the right time. He started talking to a girl he told me not to worry about right after we stopped talking and he had lied about them talking when we got back together 2 months ago. This happened with him and the other girl 4 months ago. I feel a bit hurt it was so soon after what I thought was a special time with him and that he had mentioned we needed space to work on ourselves. But again, I’d never be upset with him for this. I felt gross after reading the texts between them and wondered how things could become that heavy (heavy sexual and romantic flirting) so quickly (3 days after we stopped talking) if they weren’t talking while we were talking. Any advice on how to work through this? I have insecurity and jealousy issues I need to work through I think.

TL/DR: my bf and I broke up for 7 months, we talked in between and 3 days after we stopped talking he started talking to a girl he told me not to worry about. How do I get over this hurt feeling?