Bra sizing?

Okay so, right now the bra that I'm wearing is a little bit big in the cups, and the straps are extremely loose even when I have it on the tightest setting. If I hold the straps in a place where they're tight, the cups fit perfectly. But then the wire on the bottom of the cup is so tight that it leaves bruises and has left a scar on me. What do you guys think that I should do? Right now I'm wearing a 36C. I can't go in for a fitting because my mom won't let me. And if I could go myself don't you think I would 🙄 I can't drive