I feel I have a reason to be upset, but my partner doesn’t. He said he kept it a secret from me because, in his words, “I knew you’d react this way.” 

For one, my partner was flirtatious/asked sexual to this girl when he was younger (about 13/14). According to him he was just being “a silly boy.” And maybe so. But that proves there’s a sexual chemistry. He hasn’t spoken to her in about 3 years - then I come to find out he’s been confiding in her recently about all our relationship problems, exchanging photos and videos (non-sexual), then acts incredibly guilty and breaks up with me. All this time he’s been connecting with her while he’s been incredibly distant with me... It doesn’t seem right. He claims “they’re just friends” but considering everything, how can I believe it?