Please help, why has he done this?

My boyfriend broke up with me the other day, it was a brutal and VERY unexpected break up, because a couple of hours before we was perfect and happy or so I thought. I'm sure as you can tell, this isn't my choice. If im honest I'm absolutely broken. I'm continuing to torture myself and logged into his instagram and can see he's flirting and already arranging to meet with different girls. It's killing me. He came round late last night and collected some more of his stuff, as he was gathering the items, I couldn't even bring myself to look at him. He said to me that he's hurting too, but he seems fine. He was maybe there for 10minutes and as he closed the door he told me he loved me. Naturally I broke down. Why is he doing this? The pain is unbearable, I just want my bestfriend back.
Please help, why has he done this?
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