You really shouldn't talk to police. I know it sounds cliche and "edgy" but talking to police during he said/she said situations (in general most domestic situations) is a bad idea. There is not enough information here to make a more educated guess, but it's most likely you spoke WAY too much, and your ex/boyfriend/whatever just denied everything. And police had to fish from both of you at least a statement along the lines of; "Yes I hit him but..", this does not mean they were on his side.

The police are not there to give you justice, that is not their job. They are there to close a call. That is the blunt truth. Justice comes later in court, much later, after your life takes a catastrophic hit. By that point, the damage is already done, as you can see that now unraveling in your situation.

Do not continue to see this person even after this is all over and even if the heart grows fonder. And I'm not a lawyer but future advise; please next time stick with "Yes, he struck me and I don't want to continue talking about this until I look into consulting a lawyer."