I can't get over something that my boyfriend did in the past. Any suggestion?

My boyfriend and I have been together over a year. When we first started dating, I noticed that he started following a girl on Instagram. Her profile intro and picture looked like she would be a girl who's very self-absorbed and slutty, but since it was a private account, I couldn't see the contents of her posts.

I requested to follow her at least 3 times, but she would not accept my request, even though she had 1000+ followers, many of which were probably guys. I have a decent profile intro and picture. It wasn't like I didn't look legitimate or something (unless I intimidated her, since in my profile I mentioned my profession).

I created another account to observe her account, without requesting to follow her. One day her account became public (needed more followers?) Sure enough, her posts were all selfies of her face and body.

I asked my boyfriend about her. He said that he didn't know her, and I believed that, but my question was, why did he follow a random girl? What was his purpose? Her posts did not have meaningful captions, and there was no intellectuality nor inspiration in her posts. Moreover, my boyfriend agreed that she was slutty, yet he continued to follow her, since the beginning of our relationship.

Eventually my boyfriend unfollowed this girl, either because he wanted me to shut up, or he cared about my happiness. The one thing which left me scars and I can't get over with, is that he never gave me an explanation for why he followed her... I'm sure it was merely to look at her.

I can tell that he's tried to make me happy, as he corrected some of the things that he did and hurt me before, and I feel happy when I'm spending time with him, but whenever I'm having alone time (eating breakfast, showering, etc.), I start to think about what I should've said or done at that time, think about how I must be right that he followed her just to look at her.

I just can't get over it. It was probably about 5 months ago. What can I do? Thank you.
I can't get over something that my boyfriend did in the past. Any suggestion?
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