My boyfriend constantly likes other girls pictures?

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Okay so I'm in a long distance relationship and we have been really happy together until about 2 months ago. On tumblr and instagram he would always like other girls pictures but not mine. I have had really bad insecurity issues in the past and still kind of do. I have told him that it upsets me and makes me feel not good enough, but he still continues to do it. He might think I don't look on what he likes but obviously he can't be trusted. And I know he's a guy so he will think other girls are good looking, but it's one picture after another and sometimes of the same girls. I don't want to bring it up with him again because he knows I will have been looking through his likes.