I will try to be constructive.
Do I undertand right that you have problem with two things?
1) You are afraid of pregnancy and/or some deseases.
2) You are afraid that you daughter have some sexual compulsive problem?
1) You say that she has bf. So let her and her bf test to deseases and you can be sure that she will be clear (Be happy that she has bf and no one night stands).
To pregnancy problem - there is lot of types of contraceptuons. You are afraid that she will not use pills right. So she can be on IUD, injections or hormonal implant. Do you really think that is easyer leth her wear cb than use contraception? Really?
2) Her often masturbation. Here are two possibilityes - it is some type of compulsifity without sexual subtext or she is really really horny. We can not say which one is true. You need diagnose from doctors, like psychiatrics and sexuologists.
If she is overhorny chastity will not help her. It will be only worse and worse. In this case she nees relieve! But might be some medication can turn her of little bit. And some sexual toy like womaniser can make her cum easyer and faster so she will have more time to study.
If it is some type of non sexual compulstatory she needs terapy - a psychologist maybe a psychiatrist. In this way cb can help. BUT ONLY when she will be the only one keyholder for herslef and NOBODY will ask her to wear the belt. But before it she has to want to help herself by this way. She has to ahve acces to sex and masturbation all the time. Because the problem is not sexual (not arousal and sexuality), but the compulsivity is consequence of other problem (like nail biting, night pissing, self harm, etc.). You will not solve the situation by prevent manifestations...
