PigtailSam Let me ask you some points
1) If she's struggling with her studies, how do you believe that a chastity belt will help her? AFAIK, frustration is really an enemy of concentration.
2) What makes you believe that a chastity belt will keep his BF away? A chastity belt can be a very powerful sexual tool. What will prevent her from learning a lot of new tricks? Will you put her a muzzle? handcuffs? Frustration will increase her sexual activities.
I'm pretty sure that a smart woman like you knows that you are damaging your daughter. You are showing her you don't trust her, and that basically she failed. But you should be honest with yourself, you are not protecting her, you are just protecting your peace of mind. And if it is the case, and only you know, your failure as parents out shadow her failures.
You tried several things, but mental health is very complex, and it is a world. A parent that really wants to support her daughter will try harder, ask for other specialists, will tell her daughter "I'm with you, I'm supporting you, I will fight with you". Then, of course, your choice.