Boyfriend slept with someone else

So me and my boyfriend have been together for 3 years all the sudden he just breaks up with me and blames all reasons on me. Then he went and slept with another girl twice and told me and said If I go sleep with anyone he will never speak to me again, and now he is all like acting like we're back together and told me I just need to change and he will take me back maybe. I don't know what to do or what's gojng on? Advice much needed! Any questions feel free to ask!