Question for plus size girls (about feelin comfortable with my boyfriend.)?

Okay now I wasn't quite sure how to word my question, so let me explain. See I am going to be 18 next month, and the day I turn 18 is also going to be mine and my boyfriends year anniversary of being together. We are going out for a romantic weekend out at a cabin, I want to finaly feel comfortable showing him my whole body, but I am a plus size girl and no guy has ever seen my whole body. We already live together and everything, but I have never felt comfortable revealing myself to him.I mean he makes me feel beautiful and so sexy somtimes, but then when I look in the mirror, I feel crazy for ever thinking about showing him my body. I am 5'3 and 220lbs.

MAIN QUESTION: for other plus size girls is, do you wear your clothing on or under your gut? I know the question is really odd and I feel so lame for even asking, but right now I wear my underwear under my gut and I feel like it makes me look unproportioned.

He is super skinny probably like no more than 140 maybe, and his last girlfriend was a twig!

I suposed those two factors are why i feel so uncomfortable, along with the fact that i have no show my body b4.