How do I get back money that i loaned to my ex -boyfriend?

I let my ex-boyfriend borrow 1200$ on Wednesday, he told me he would pay me 400$ every Friday for three weeks. He broke up with me two days later. What should i do? That money was what i had saved up to buy my first car. I found out a few days later that he slept with his ex on Thursday and has another girlfriend right now. On top of that I only have half of my rent. I don’t know what to do i need help.