I'm in the same position right now, having gained some weight over the past year. I find that bringing a really close friend or a significant other along when shopping helps me. Pick someone super close to you who will be honest and whom you can trust to love you and support you through whatever reaction you may have to the experience. Crying in a dressing room sucks, but for me, it's way better when I have my partner there to help me get my shit together/tell me I'm still beautiful/remind me that sizes mean nothing/etc. It's also really helpful when you're in a dressing room with intentionally awful lighting (Feel bad about yourself! Buy our stuff!) to be able to ask "do my thighs really look like that?" and have them answer truthfully.

On that note, it's important to remember that so much that stores and companies do is meant to highlight your insecurities and get you to buy their products. Dressing room lighting, vanity sizing, mannequins, only carrying one XL in the shirt you want and hiding it wayyyy behind all the others, etc. Try your hardest to ignore the size on the tag and just go for what feels and looks good. For what it's worth, I have pants from sizes 10-16 and other clothes from M-XXL that all fit me. Go figure.

If you end up fitting into a bigger size than you thought or want, google "size ____ women" and check out how gorgeous they all are. This is such a weird piece of advice, but helped me realize that a) I wasn't actually that size, it was just a specific pair of shorts with weird sizing and b) even if I was, those ladies are hot as heck and it wouldn't be a bad thing to look like them.