Why am I afraid to be alone? I don’t deserve this

My BF (41) and I (31F) have been together almost 4 years. Lately we have had a lot of fights because of emotional unavailability, feeling like not a priority etc. so I kept having dreams of a woman. And I finally went through his ipad and saw messages from him and this girl at his old job. They were from back in September. A few messages. So he says “Good morning baby” and tells her I miss you. We get into an argument. He deflects and yells at me for going through his stuff. And try’s to say I’m the one cheating probably. He then begins to pack because he slept over for the weekend. And so I helped him pack to get out. When he seen I was serious he starts to say I’m sorry. I should’ve told you blah blah. Then he says she was someone I was flirting with “work wife” back when I worked there. This was before we were together. I’m like ok why are you still in contact with her. You don’t just reach out a former co worker randomly and say good morning babe or I miss you. You said back then she had a bf so you didn’t pursue. So what you were checking back in to see if she still had said boyfriend. He then says oh I told her I had a gf and she said she had a bf like that made it any better. Oh we were going through things. B.s. you guys were conversing before this exchange, something is deeper here and I don’t trust it. Anywho he keeps saying I am not ready to lose you. Like too bad. You’re a liar. You acted like you had no time to cheat or blah. You acted like you cared, I did all these things for you including watching your kids when you had to work.this event happened after I did all these nice things for your birthday. I feel like I’m foolish for even trying to forgive and make it work. I think I just need to let it go and stop focusing on the lonely part