I've been dating a guy on and off since I was 12 or 13. We've had a lot of really good times, but a lot of obstacles as well. He's been around for a very long time, and I grew very attached to him. We broke up for the third time about a month ago. The relationship hadn't been healthy, but he fills my thoughts more often than anything else. I feel really alone, and I just want my best friend back in my life. It isn't so much the break up that's bothering me, it's some of the things that happened while we were dating that I can't get out of my head. I have to go to therapy in a few days and I'm extremely nervous. I've been very depressed (it was partially a reason we broke up) and I don't want a psychologist to omit me for severe depression or anything.. I feel like I can't really let the relationship go..

You have to tell your therapist about this... it matters in the big picture.

Its been a month... its still fresh, you have to give it time... in 6 months if you haven't got past it then its time to stat thinking of therapy for it... if you goa year or more then its definitely time.

But after only a month... its normal and its still going to be pretty raw... and it would be for almost anyone.

Think of it this way... whatever happened is the past... consider everything negative as one of the reasons you finally woke up and decided enough was enough. There is usually a lot of things the other other person did before you decide to call it quits.

How old are you now?

You meant to say commit, not omit, I assume.
Here's how it goes in therapy if you are thinking about suicide:
The therapist asks for a 'contract' with you, to be open and honest about whether you think about
just wishing you were dead but can't really think about acting on it,
wishing you were dead with thoughts of killing yourself,
wishing you were dead with thoughts of HOW to kill yourself,
and finally, actual plans to kill yourself.
The last one lands you in an ambulance, by law and policy. It's OK to talk and talk about the previous ones from session to session, and of course if you are in agony about real plans to kill yourself but keep putting it off, then talk about it anyway. In fact there isn't much point in therapy if you can't talk about EVERYTHING.