My boyfriend of 2 years went away on exchange for 4 months. About 2 months in, I broke up with him because I was feel so good here by myself. I was doing a lot more and I felt like I was living for me. About 2 weeks later I knew it was the wrong decision but decided to wait until he returned to tell him.
He came back a few days ago and we met up for coffee. I poured my heart out, telling him that I still loved him and wanted nothing more than another opportunity. I told him that regardless of the past this was where I was now and that I was going to fight for him, but would respect his decision if he asked me to stop.
He then told me that he had been seeing someone the last month he was away. It hurts like nothing ever has before. I don't know what to do.

I then rearended someone driving home. I feel so guilty and stupid. I'm a new driver and I just feel like such an idiot.

Wait, so he was seeing someone else while he was gone BEFORE you broke up with him? If so then you made the right decision leaving him. He obviously doesn't care enough to stay faithful. Don't worry about him, he lied and betrayed you. As for the crash, you had a LOT on your mind and made a mistake. Plus you said you're a new driver so it's nothing to worry about I don't think. Let him go and find someone else. You'll get better I promise, I know how it feels trust me.. Try the stikes and looking at some of the questions already on here about getting over a relationship on here too. Good luck!

He was seeing someone else after I broke up with him, he didn't cheat. In his words, he was in a foreign country without friends and support and needed to find support somewhere. I don't want to let him walk away, I love him so much!

As I understood, he started seeing this other person AFTER you broke up with him. Could be just a reaction to the break up, a rebound. But who knows.
You know best if it's worth fighting or not. Wait a bit until it all settles down, you don't have to repeat how you feel. He knows now, and he has to 'choose' on his own.

Don't move on until you're sure you need to and want to. Relationships are tough, break ups are tough, moving on is the easiest solution quite often. You broke up with him, it wasn't easy for him too.

You have laid it out, and he was honest. You don't have to do anything until you calm down and get yourself under control. Then see what happens.