You and your lover egged each other on, and not in a good way. Apart you were able to focus your energies on a goal and reach it. Together you were apt to indulge yourself in all sorts of unhealthy ways.
You may have run up large credit card bills, or been in some other sort of debt. You acted too quickly, you moved without thinking, you leapt before you looked. Other people may have been coming to you to express concern -- your life wasn't moving in a positive direction, and though the fault wasn't exactly all your lover's, your relationship certainly didn't bring out the best in you. You two needed to set up some ground rules. You never sat down together and talked about where you were and where you want to go. What were your career goals? Where did you see yourselves in five years? In ten? You needed to articulate your dreams, and make concrete plans that would bring them to fruition. Discipline and restraint wouldn't have been easy, it's true, but this was the only way to combat your drift. Letting things go as they did only lead to moving further and further apart. Next time you'll know to make more of an effort to keep things together.