Obviously, the fact that the money ended up in your account does point to you as the person who did it and investigators are likely to focus on you.
However, accessing your ex-boyfriend's new account would require knowledge of his account number and online password. Is there any reason to believe that you would know these?
You do not have to prove you didn't do it -- the prosecutor would have to prove that you did do it. However, if you are unwilling to identify the person who did it, that would weaken your defense if the case is prosecuted.
Why would you want to protect this other person at the risk of being convicted of a crime yourself?
The amount of money involved would probably result in a misdemeanor
prosecution and a sentence
to probation and a fine, though either a small jail sentence or alternative thereto (such as community service) might be imposed.
Even a misdemeanor conviction for a crime like this, however, could have long term adverse consequences (such as preventing you from obtaining certain kinds of state issued professional licenses and preventing you from being hired for certain kinds of employment). You should give serious consideration to whether you really want to accept such long term consequences for something you did not do.