I see two issues here. First if you had told the truth had your boyfriend done enough to get arrested anyway? You are ambiguous about the knife; throwing a knife that lands on the floor could be seen by the police as a threat. In other words your exaggeration may not be the reason he is in jail. Second, of course you want to come clean and tell the truth, even if it won't help him that much. There is usually a victim's advocate in the DA's office to help with these problems. I would speak to that person first before I denounced myself to anyone as a liar. Explain that you have had second thoughts and talk to him/her candidly about it. You have another choice and that is to tell the DA you are not going to cooperate and recant your story if forced to testify. This may work but it also may not discourage the DA. The DA, if he's honest, proceeds on the evidence. If he sees evidence of abuse, the fact that you feel bad having called the cops is not going to deter him.
Unless you are truly a victim of domestic violence (and you may not be willing to accept it if you are), I advise people to leave the police out of their domestic disputes. Once you invite the cops into your living room it's out of your hands.
Some people are under the impression they can call the cops on a partner to teach him a lesson and then drop the charges. Once you call the cops you don't press charges and you don't drop them. The DA does. Everyone should think twice before opening their home to the cops.
Mr. Kaman is absolutely correct, as usual. Make a choice from the suggestions he makes, or contact a local criminal defense lawyer personally to discuss your options. Good luck.
This response does not constitute legal advice. Given the nature of this website, it does not create an attorney-client relationship. This answer is provided solely for informational purposes, for you to use as a starting point when speaking directly with a lawyer in your State. I urge you to immediately contact an experienced criminal defense lawyer admitted to practice law in your State before you make any decisions about this case.