Advice please

So I’ve been in my relationship for 6 years, but he wanted a break last year because he said we got together so young and he wanted to be single. Well.. he slept with a girl and lied about it when I asked if he was seeing someone else (cause I wasn’t going to continue doing what me and him were doing if there was someone else) well.. we eventually worked on it and got back together in February of this year. But I’m so insecure now. How do I overcome this? He’s always on his phone and I just feel like there’s someone else again and I feel like I’m not good enough. He literally said we had no issues and he still loved me and we never fought but he still wanted a break. Like it’s been a year already I just want to feel good enough and lovable. He always compliments me and tells me he loves me but I guess I’m experiencing doubt? Like at any moment it’s just going to shatter again. I picked up a gym membership and am working on myself in hopes that it’ll help with how I feel. I just look in the mirror and I don’t feel good and I just want the old me back. Any tips or advice?