We broke up a month ago, he has already fallen in love with someone else, should I ask him back or let him go?

I have had some discussions here about my ex boyfriend, I did end the relationship and moved out because of his dishonesty. It's been a month and we were still dating and he promised to change and he was going to be true to me and try to work things out with me. I now found out he spent the weekend with another woman and he still won't admit it to me, but I read his emails and after knowing this woman one week (8 days) he is professing his love for her. He told her he's been single for 2 years, no mention of me and us, he met her online (like we met) and now after their first date he's telling her he loves her and he has stopped calling me. Every time I trust him a little, he disappoints me but I feel so attached to him. I just need some thoughts on this. I know I should move on but I am having such difficulty, it's extremely lonely and my heart aches deeply. We had an intense 3 years together and we were together all the time, I truly wanted to believe in him - I always have mixed emotions because I know he's wrong for me but I love him so much and I don't know why. I think I'm afraid to be alone because it's the first time I have ever been on my own, totally on my own. I miss him and it hurts to read all the things he's writing to her, it's all the very same things he wrote to me. It's like he transferred his feelings over. It's unbearable and I stopped reading it but it's devastating that he can move on so quickly when I am here hurting so much.