You were right to break it off in the first place. Remember how it felt to be neglected? How it felt to come second to his career?
Now imagine having that same feeling 5 years from now.. 10 years from now.. 15 years down the road. Is that the kind of life you want?
Yes, it hurts being apart and breakups can take months or years to heal, if ever! But does that compare to the pain of feeling neglected on a daily basis for life?
If you want to stay with him, then you need to make some rules. You need to tell him what you are expecting from a relationship, and ask him if he's looking for the same thing. Then give it a test run. Tell him and you can continue to see each other for 3 months (for example), but that you are not officially back together. Tell him it is a test, and if he fails, there are no second chances. Give him a chance, but don't fall too hard.
People can change, but they usually don't. How much pain are you willing to let this guy inflict on you? Remember, that's your choice.
And good luck. My best friend is still sleeping with a guy that broke up with her over a year ago. It is destroying her from the inside, and while she knows that, she just can't cut ties. It's not easy, I know!!