It wasn't fair of your friend to pressure you into doing something you didn't want to do. That was selfish of her.
Although, it sounds like your friends really want you to be there, and enjoy your company. Good friends don't judge you based on how you look. People who judge you on how you look don't deserve to be friends.
I do understand feeling self conscious, even in front of non-judgemental friends. You are 4 weeks out, and there is a lot you can do in 4 weeks if you want to. But not because you want to impress your friends or keep your husband's attention - just for yourself.
You can get a lovely haircut, revamp your make up 'look', get some new clothes that make you feel good, start a diet and exercise plan, think about old hobbies you'd like to revive or new ones you'd like to start, maybe enrol on an evening course, start a daily walk, join a hiking group. Things that will make you feel better about yourself, improve your health and fitness, widen your social circle, and help you to stop focussing on the physical.
In the meantime, if you don't want to go on the holiday, you don't have to. Be honest with yourself, and prioritise your own feelings over your friends' feelings (any good friend would totally understand that, given the headspace you're in).
And finally, you deserve better than your DH. Perhaps if you start on a journey of improving how you feel about yourself, you might decide that you're better off without him. But you don't have to think about that right now if you don't want to.