I have to go to a wedding next week and I'm freaking out

My sister's wedding is next week, and I'm freaking out

This coming up Friday, my mom is planning on driving me to go shopping for clothes, since I don't have anything formal to wear to the wedding (I wear sweats and a tshirt underneath a hoodie every single day). I self harm, and I'm so scared she's going to get upset at me if I ask to buy a long sleeve shirt, since whenever I try to do that, she argues that it's too hot for it.
I don't know how to convince her to let me wear something that isn't short sleeves. i dont know if she'll even listen to me. im so scared. im going alone shopping with her and cant stop crying from just the thought of it. i dont know what to do im so bad at talking to her i usually just fold whenever she tells me to do something. last time we went to get clothes, she argued with me because i wanted to look for jeans, and i was so overwhelmed that i ended up not getting any jeans at all.

im terrified and upset and dont know what to do. i dont know what to tell her to convince her. im bad at coming up with lies that sound convincing but i cant just tell her about my scarring. my friend says to be upfront and tell her its because short sleeves will make me uncomfortable (without specifying why) but i dont know if thatll work. please someone give me some advice. im panicking