This is a very sensitive topic. To clarify:
You called the police on your boyfriend because you were afraid he would leave you? Furthermore, you're saying this isn't the first time where you've let him be arrested for domestic violence you initiated?
I am not a lawyer. I think it would be best to speak with a criminal defense lawyer in your area and explain to them your situation. He or she could properly explain the laws in your state as it pertains to your domestic violence case. I've read that in some cases courts won't just let a person to simply use their fifth amendment privilege with nothing to back it up. The court may ask you for more information through an attorney as to why you wish to plead the fifth. Again, an attorney would be able to explain all of the legal stipulations to you.
Although I could be wrong, I would assume since you broke the law and lied to police when you said you were hit first, you would be able to invoke your fifth amendment rights to avoid incriminating yourself. In most cases, however, a person cannot invoke their 5th amendment rights to avoid testifying. Depending on the law in your state, if you refuse to testify, any statement you made to police when your boyfriend was arrested might be able to be used in court.