Am I just being really paranoid?

Been with my boyfriend 4 months, everything's been great, except about a month ago he started speaking to this really beautiful girl he met online. They liked each others pictures (she did first, he said he did it back just to be polite) and she asked him to add her on Facebook, which is where they now talk regularly. She lives at the other end of the country but it still worries me that they're speaking because since he began talking to her he's gotten secretive with his phone, before he used to let me do anything on it (and vice versa, which I still let him) but now he doesn't. He still lets me see his screen when he's on it, he just doesn't like me touching his phone. Recently I saw a message from this girl and she gave him quite a lot of kisses, about 8-10? I know I'm probably over thinking this (I overthink everything) but I got quite upset so I asked him about it as I was already worried about her before, and he said he just gives her kisses back to be polite and they mean nothing to him. We never do kisses when we text, so it bothered me because she gets kisses and I don't. He told me he loves me and only likes me but I just can't get the idea of another person out of my mind. I've been cheated on by every boyfriend (3) in the past so maybe I'm just extra wary of it but how can I stop myself getting worried about this? I mean clearly he wants to be with me or he would have left by now. I keep mentioning things and it makes my boyfriend angry and causes arguments so it is affecting our relationship, it also makes me clingy because I want him to be talking to me and not her which isn't good either. How can I just let it go?

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