Long distance relationships are very difficult. I met my husband online and we were apart for a very brief amount of time. This was back before facebook,etc. We met in a chatroom and I can totally relate to the how seeing another person paying attention even though you agreed to be poly would be kind of difficult. I think you were right to delete your accounts.
I would probably not be hasty since it sounds like you have feelings for this guy. However, I know your studies should come first. How hard would the visit be if he was able to visit? As far as your studies go?
I can't say that I did even feel a little twing of jealousy over something a friend just recently said about a mutual friend of ours. I was surprised she said she wished she could have a booty call with my friend, and it came out of left field for sure. I certainly could begrudge either of these people some happiness, but I wouldn't want to know about either, so I that's where I relate to your pain in seeing others interested in your guy.
I guess I have no words of wisdom but I can definitely relate to your quandary. I hope you will not neglect your studies though. You have come this far, and it is only another few months.
Best wishes to you.