A Breakup during lockdown

My partner and I just moved in together In February. It was very stressful as this is my last semester of grad school and I had to give up my two cats. This lead to depression and a lack of intimacy. When the lockdown started I decided it would be good to spend some time apart. This way I could focus on my homework and not the strained relationship. We were apart for two weeks and ended up breaking up over a text message debate.

the day after we split up he met a girl online and started chatting. I moved my stuff out a week and a half after the breakup. When I arrived to get my stuff he informed me he had plans that afternoon and needed me out at a certain time. his plans were to have his new 'friend' stay with him for a few days. They met the day after we broke up and spent hours on the phone together each day, exchanged cute photos and said goodnight and good morning every day. Only two weeks had gone by since our split.

I didn't know all this at first. The weekend after I moved out, I saw him and said maybe we rushed moving in together, as well as the breakup: and maybe we could try again. He told me about the other woman and how she was upset he was even talking to me. I couldn't believe it. How could she have any authority when it's been two weeks? He and I want to get back together and forget it all but I can't forget it.

how do I stop being insecure about the rebound that he had and feel that our connection was special? How could he be so intimate so fast?