I am getting married next month. I’ve put on a bit of weight and thought I could lose it in time but it hasn’t happened. I do look fat: there’s no way I can avoid that.
Ok, being realistic, there's nothing you can do here, you say that yourself. You've already decided that.
What you could do, for this one off event, is crash diet. A month is plenty of time. It depends how much weight you'd need to lose in order for you to think you don't look "fat" and only you can answer that. If you've put on half a stone, that's doable. If you've put on 3 stone, it's not.
Stupidly, I thought I’d get someone to do my hair and makeup. And I had a trial run yesterday and it looked RIDICULOUS! The hair style made me look even fatter and I looked like JK Rowling’s description of Dolores Umbridge. The makeup just made me look stupid. There was nothing wrong with how she did it, it just looked silly on me
So, one person did your hair and make up and you didn't like it. You haven't signed a legal binding contract that she has to do it again, or you can't get married. Just do it yourself. You've paid her for the trial, as you should. At least you found out via a trial rather than on the day when you would have no opportunity to change this.
I paid a make up artist to do my face as a treat once when I was going on a date. I looked the ugliest I think I've ever seen myself. She just got me all wrong. Sounds like you've had a similar experience. So find someone else, and if it's money that's the issue, then do it yourself. A month is enough time for you to watch some tutorials and practice on yourself in the evenings. If you had a go every evening, you've done 30 "trial runs" on yourself and will know which looks you like best.
I think you sound really defeatist, and I do understand why you're upset, but don't agree there's nothing you can do. Either resign to the fact (which is what you already appear to have done) or do something about it.