💐 you're not nasty. The bastards that abused you are nasty! Be kind to your inner child. People hurt you and so you want to hide your body & protect yourself from being hurt again. It's a normal reaction. It's good you're getting therapy, but it's hard. You're dealing with so much 🙁💐
I think you need to stop worrying about your weight. Focus on getting well, both emotionally & physically. I think 'a diet' might well push you over the edge right now.
Try to eat well, sleep well & move. Try to get fitter so you enjoy your holiday more and try to find things other than eating that comfort you. It's not easy I know, eating shoves down other feelings.
You are presumably flying with your family, so you'll be fine. Sit between them, or on the aisle with one of them next to you and you'll fit just fine. The extension seat belts are huge and with family next you, you, it won't be a problem.
Where are you going?