I feel sick, why can't I trust people?

So I'm bored, I scroll on my Instagram, see a picture I tagged my boyfriend in, click on his Instagram, saw a picture of a wine glass (which was posted before we were together but obviously I didn't know that until I looked) so I clicked on it thinking "I don't remember him taking a picture of any of my wine" and then saw it was posted before we were together, noticed it had a few likes on it so I looked to see who liked it and this girl popped up who looked his type and instantly my stomach dropped, I have literally only ever been screwed over by people I trust so I have a very hard time with it. I clicked on her profile and then one of her selfies and he had liked it, I clicked on another one and he had liked that one too, this happened with a few of her pictures, all selfies and all uploaded while me and my boyfriend have been together. I feel like I'm being so dramatic but maybe I'm not and I just don't know.

I went onto her Facebook profile and it says that he's friends with her, no other mutual friends (I'm friends on fb with his friends and family). He also has his friends as private which makes me feel even worse.

What do I do?