BF is splitting up with me over FB message how can I get him back please help?

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Me and my boyfriend have been together on and off for 5 and a half years since we were 16. When I was 18 we briefly split up and I started seeing someone else which cut him up. Then he did the same to me which I forgave. a year later during summer we split again and I started a sexual relationship with a guy. I was friends with him on FB but he asked me delete him which I did. But when he went away to NZ for a year this guy friended me again and I accepted as FB means nothing to me.

We never spoke on it and then 2 months ago he wrote to me one night here is the conversation:
HIM:
Hey how's things random I no but do u want to meet up for a chat..could do with talking to someone that I trust.
ME
Hey yea ok why what's up

HIM
Feel like an ass writing to ya

ME
Why what's wrong ?

HIM
Just me been me keeping things bottled up

ME
What happened?

HIM
Nothing really just been down the last couple of months do no what really to say

end of conversation. I never met him nor had intentions of I just wanted to know what he wanted and what he was going to say. I left my phone powered off with my bf when I went to the toilet one evening we were out and He got my phone turned it on and went through all my msgs and found this. Now he is accusing me of cheating on him which I am not and have even given him access to my phone records and ring him off my house constantly to reassure him and he is calling me a compulsive liar and said he wants nothing to me with me I am only cancer. I love him and I am terrified of loosing him. I know I was wrong not to tell him and to talk to this guy when he asked me not to but I really didnt see it as a big deal as the conversation was jus BS. Please help me what shoud I do to get him back? will he come around