Boyfriend commented on other girls' instagram account... how do I get over it?

I do not have many social media accounts. I find that if I want to share details/photo of my life, it is often with someone in my close circle of friends.
My boyfriend has (had) Instagram, as several people do. When we first started dating, he would mention that his cousin's girlfriend would post a lot of half-naked pictures of herself on her account. He said he was 'grateful' I didn't do this kind of thing. He didn't mention her name a lot, but I guess he mentioned her enough times that months ago I searched his instagram username and found he had commented on one of these half-naked pictures. He had also commented on other girls' pictures too, but this particular picture bugged me more, because he knew her. The other issue is, I've told him I was hurt in seeing comments on Facebook before, and he stopped. He didn't stop on instagram, I guess, because I couldn't 'see' what he was saying.
I confronted him. His story changed. First, he said he commented because she wanted to have more followers and asked him to like and comment her photo. Then (about a month later) he said he noticed a lot of other guys commenting/liking the photo so he commented to too 'bug' his cousin (however I definitely recall his was the first comment). He said he doesn't know why he lied before.
I told him I was hurt and felt deceived. My thinking was, why lie and make jokes about her if you actually find her attractive? He denied liking her. He has since deleted his account (his decision).
It has been months. I rarely bring it up but find myself hurting still. I know that this isn't fair to him either.
He sees her. He will always see her. She just had his cousin's baby. In a way, she's part of his family now.
How do I move past this hurting?
Boyfriend commented on other girls' instagram account... how do I get over it?
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