I've always doubts about unbreakable chastity belt,as there is no unbreakable chastity belt.It was already discussed ,and it was some fetishist who fantasize about the idea of being locked in chastity belt,with no way to escape it(or similar stories).
But I would give you the benefit of the doubts.
However,I don't think anyone here have cut her belt;but the posts of @BikelockFugitive(our martian troll! 👹 😉) can give you some hints to pick the lock or destroy the belt.
If you don't want to do it alone,or find it too hard,go ask for help from firemen.They would probably manage it easily,and you could tell it is a game between your boyfriend and you,but he lost the keys and you were locked in this,if you don't want to put your parents in trouble.
Anyway,you should not be forced to wear it.You should be allowed to tell her to give you the keys immediately.Even if it could have consequences,like no financial support,breaking the relationship with your family,like it is for some of us;but we could still decide it is better that wearing it,and accept the consequences while free of the belt(which could happen anyway if you cut/destroy your chastity belt).
You plan to cut your belt too?
Renita It's only result you can achieve doing it yourself, e.g. bunch of injuries and the belt is still in place.
Read the post form @BikelockFugitive too,he has given some tips,if I remember well(using gallium or something similar,to destroy the belt)
Renita Beside it, it's not something unique, every belted girl had it on her mind someday.
mekekelo I doubt you will "lose your parents", most adults shouldn't hold a grudge over something like this and should be able to forgive you soon enough.
Or when she marry,maybe.