I'm having some issues in my poly relationship

Elli • Elli~ 19~Midwest but east coast raised~lover of memes and dogs~wiccan~self-deprecating hipster
So about six months ago I started seeing this guy, with the knowledge that he already had a girlfriend. He devulged very early on that he was poly and in an open relationship with her. 
At first me and him didn't define our relationship and for the first few months he continuously called her his girlfriend and talked about her at some point almost every time we saw each other. We only ever defined it after I had an old fwb hit me up and I had to tell him I had a bf to get him to back off. I told my partner this and he was perfectly okay with it and since then we've continued to have a defined relationship. 
The other girl knows about me and they've been together for about 1.5 years. At first being in this relationship wasn't too hard, I had some jealousy issues that I worked out but it was easy to overlook my negative feelings because he still managed to make time for me. 
Over the past few months though it's gotten exceedingly more difficult. I feel like I'm playing second fiddle. He sees her almost every day and they go out on dates and spend nights together constantly. My parents are strict and neither of them live with their parents. They both live in apartments off campus. 
Sometimes I feel like I'm a secret girlfriend. She posts about him on social media and takes pictures of them together all of the time. He's told me that he hates it when she takes pictures of him because he doesn't like people taking pictures of him. That and he isn't the biggest fan of social media. 
At this point I'm not sure what I should do because there used to be more equality but now that my schedule is more rigid I almost never see him.