My bf cheated.. I need an outsider opinion

So ive recently found out that my boyfriend of 2 years has cheated on me. First off, we started in a long distance relationship (for a year and a half) and then he moved in with me for 6 months (recently broken up). Now, first off i found sneaky texts that were saved under one of his friends names which actually was his ex girlfriend where they both were telling each other how much they love and miss each other (now that he had moved across states to be with me) and also met up with her on multiple occassions without telling me - he is justifying this as comfort as her father passed away while they were dating and he is the only person she can speak to about it, which i understand but there was no need to talk to each other the way they did. Both my boyfriend and his ex gf swear it didnt mean anything and that they never physically cheated. We broke up but then I then forgave him and we moved on with our lives together. About a month after this, I went through his phone as there was no trust between us and i found an old conversation between him and another girl and she said "i wish we didnt have sex last night" and sent a picture of me because she had just found out about our relationship. From there he admitted that he had sex with her ONCE until i got a hold of her number and called her and it was actually a 3 month affair.. now he is non stop contacted me trying to get me back and apologising profoundly and saying how it will never ever happen again and he would never want me to go through anything like this ever again. Our relationship improved 100% once we moved in together and this all happened while we were in a long distance relationship. I obviously still love him as im considering taking him back because he keeps telling me how much things are going to change although i want an outside opinion. Has anyone else been through something similar? Or are there any success stories out there? im so scared because "once a cheater always a cheater" Please help!