What is the difference between a best friend and a significant other?

To clarify, I'm not asking because I think someone likes me or that I like someone or that I'm confused.

Here's my situation:

I have been with my girlfriend for 10 months now. Our summer was very stressful for her because she was worried she wouldn't find a job before we moved and that she wouldn't be able to afford anything or be able to do anything for the rest of her life, then when she did get a job, there was a bunch more stress because it was a really crappy summer job. THEN, she left that job and become even more stressed about the same reasons above, but even more so. She finally landed a job that is good and good for her.

I'd say all this stress started up about the time we were getting out of the honey-moon phase of our relationship, so we didn't really have a chance to bond as simply best friends.

SO, we want to try and work on being better friends than we were when we met. The problem is, neither of us are very sure how to do that while being able to detach that we've known each other really well for the past 9 months or so.

What are ways to function as friends even though we've been in a relationship for many months?

Thanks all for your responses. I know it's a tough question.