The first thing you should do is find an attorney. Do not speak to the police without that person.

The next thing you need to do is realize that physical violence in any degree by either person in an adult relationship is utterly unacceptable and a sign that person both needs to leave the current relationship and has nowhere near the level of emotional level of maturity necessary for a normal adult romantic relationship.

That is hard to hear but you, and she, really should be in therapy if as adults you cannot be in relationships with other adults without hitting them.

That is a minimum expectation for children who do not like eachother, and if you cannot meet it as adults who even like eachother its a problem.

So, don't continue with this ****. I know that is hard to hear, but the reality is that if you get into another relationship not resolving the problem that actually leads to violence, you are very likely to end up back on this sub asking about this again from a seperate incident.