Ladies, I need your help and opinions!

My bf and I have been together for almost a year.  Recently, I've had doubts about his feelings for me.  One reason is because I think he still has feelings for a long-time friend of his; he still talks to her occassionally via text and I think through facebook. A few months back, I made it clear that it's kind of weird to me when he "likes" her insta pics where she's dressed in sexy & revealing clothes, and also pics where she's doing the splits and showing off her other flexibility "skills".  He said the only reason he "likes" the ones where she's doing the splits and stuff is because that's stuff she's been "working on" and that's his way of showing his support for her.  I asked him to help me understand his "relationship" with her, and maybe that stuff wouldn't bug me so much.  He explained it as they've just known each other for a long time.  (Since that conversation, he hasn't "liked" any pics she posts like that).  But I know that he had told her not too long before we started dating that if they didn't live half a country away, he would want to see how things would go with them (I don't think he knows that I know he made his feelings for this girl clear to her).  I have told him that if he has feelings for anyone else, then he shouldn't be with me because he would still be thinking about that person a lot; which, in turn, would constantly make me question his feelings for me...and that's not healthy for any relationship.  Back in November, they exchanged Halloween pics, to which his reply to hers was,"You look great, as always." (I just found out about that recently).  So it's come to a point that I feel like he's not been completely honest with me about how he feels about this girl; I feel like I dwell on it a lot.  None of his other past relationships bother me, I guess because he doesn't really talk to any of them or have any sort of relationship with them anymore.  I care about him a lot, but I don't know what to do or how to not let it get to me.  I hate feeling like I'm being lied to and/or taken advantage of.  And sometimes I don't feel like his gf, but just another one of his (girl) friends;  so maybe he's just already become complacent in our relationship. Am I just being completely ridiculous and making something out of nothing?  Has anyone else experienced anything like this?  How can I get past this?  Our relationship is doomed otherwise.