Take a chance on someone new or give my ex another chance?

Hello, I've been struggling with an issue for the past few months to the point where I'm constantly stressed and preoccupied by it, I really need some advice so I can finally decide what to do.

I'd been in a relationship for 5 years and it had some extreme highs and lows. When things were good, they were amazing. Great chemistry, lots of fun together, truly clicked and understood each other.

But that worked against us, too. He was very emotionally immature and would get angry often and over little things, and knew exactly what made me tick. He would use that to get under my skin and wouldn't stop until he was sure he had hurt me (not physically, but emotionally). I caught him in some major lies and just couldn't depend on him, he wasn't very reliable at all. Add that to him taking his sweet time finishing school (8 years to be exact, as he kept changing his mind about what he wanted to do) and still living with his parents (at almost 30), and I just felt stuck! Our relationship wasn't progressing and I felt he didn't respect me or treat me very well.

So I decided to take some time apart and met someone else. The kind of guy I always wished I had, someone who was responsible and respectful and loving, who had his own house, career, etc., who wanted a future with me! I'm very attracted to him and enjoy being with him, but here's the kicker...I can't get my mind off my ex.

My ex is doing everything he can to win me back and to show me he's serious about us, from moving out to finishing school to seeing a counsellor to going out of his way to do sweet things for me. I know I scared him and I know he wants to change, I just don't know if he's capable of doing it. I miss the way I'd laugh with him and how we could joke the way we couldn't with anyone else. I miss being around him...when things were good, of course. I think of all the good memories we made and how long we'd been a part of each other's lives for.

I'm in such a dilemma because I know I still love my ex, but what if I give it (yet another) try with him and end up missing out on the guy who is right in front of me? Do I risk it? Or do I let him go and give it a real shot with the new guy? I'm not getting any younger and feel my clock ticking...I want to be married and have kids and I know my window is closing for that to happen. I'm just so scared to make the wrong choice and regret it later.