Emily,

how disheartening to see your boyfriend's reaction to you telling him why you're reluctant to visit the family. I recently had a girlfriend leave me, citing this issue as one of the main reasons. Ouch!
Have you ever considered renting a hotel room near the flat, to make trips possible? I know it's awkward when it comes to explanations, but if you can make it work, it's worth it. You can spend time with the family without being bloated and anxious and moody. I've done it. Of course that can be a logistical dilemma . . . how to get to/from the hotel, and will your bf be there with you . . .