The District Attorney decides whether or not they want to press charges. They would likely consult with your ex-boyfriend and get his opinion but the final decision would be made by the District Attorney. It sounds like the DA previously declined to prosecute. They are unlikely to change their opinion unless they learn some new information. In other words if additional allegtions are made against you they might be willing to re-consider based on new information. Without knowing all of the facts of your sitation its hard to tell you exactly what could happen. The best thing for you to do is contact an attorney.
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He can try but he isn't the one who makes the decision. The prosecutor decides whether or not to charge and based on what you describe has happened thus far it doesn't seem likely that they will pursue it. If, however, they do charge you contact an attorney ASAP.
There is a difference between (1) Charges being dropped and (2) No charges being filed. Sometimes people confuse the two.
The DA can not file charges but later file them. If the conduct is misdemeanor conduct - which sounds like this might be, then the DA can only file charges once. If they dismiss the charges then the charges cannot be refiled. That is the rule in CA.
It sounds like you went to court and no charges where filed. The chance the DA would now file charges is very slim. They have 1 year from the incident date to file charges.
IF IF IF they file charges - then please ask for an attorney. Do not give a statement to the police. Hire an attorney if you can afford one, if you cannot afford one, then ask for one in court.
The above information does not establish an attorney client relationship nor is it meant to provide legal advice.