I'm sorry you had to wait a little while for a response. It's good that you were so detailed with what you are experiencing. First, I will say that no one person but a trained Psychologist or Psychiatrist who has spent time with you can make any accurate 'diagnosis' of what you are experiencing and how to handle it, but those of us with life experience might have some feedback that may or may not give you an additional perspective.
First, I do believe that your feelings are fairly normal for someone getting ready to make not only a huge commitment to another person but, more importantly, leaving everything he knows behind, and starting his life over, for her. I think it is natural to have apprehension about how successful you will personally be in the transition, independent of her; and this is an appropriate time to consider "what would happen to my life if we were to not work out?" This doesn't mean your relationship is rocky or that you love her any less; it is an indicator of positive self-worth.
I think that you are experiencing anxiety more than depression, however, there is likely a good chance that transitioning to a new life and leaving all that you know behind (putting all your eggs in one basket, so to speak), may cause some depression symptoms until you fully adjust, and this may be a good preemptive strike.
You are unable to take Serotonin as a pill for anxiety, so did you mean Xanax or Valium or Klonopin or something like that, possibly? Serotonin is a neurotransmitter chemical in the brain that the neurons have trouble with sometimes. SSRI antidepressants help the neurons work better so the brain gets more Serotonin. Wellbutrin is a fantastic medication, in my opinion, though it does not work on Serotonin, it works on two others, Norepinephrine and Dopamine. Dopamine is the "happy" one. So many people spend years only trying antidepressants that build up Serotonin (including me), not knowing there are other brain chemicals that may be just as, if not MORE important to their chemical makeup.
For what it's worth, I disagree with your friends when they say that your feelings for your girlfriend will become this or that when the Wellbutrin "kicks in". The drug doesn't have your girlfriend's name on it, you know what I mean? For all you know, your feelings about the relationship may completely change when your brain chemistry balances, and you're able to see things you weren't able to perceive before for what they really are. Of course you don't want to hear that, and doubt only causes a spike in anxiety, and for that, I'm sorry. I want to share a realistic viewpoint with you.
The most important thing to do at this juncture in your life is to step out, get alone, get away from the cacophony of noise of the city, let your body calm itself, fall into silence, and feel your intuition meander its' way into your consciousness. If it doesn't match up with your "plan", don't panic and reject it, just take a deep breath, start over, let your heartbeat slow down, and listen to your gut.
It's okay to be afraid. I would be SO afraid if I were doing what you are because that is a HUGE life change! What is right for you is already inside you, though. It's instinctual; you just need to listen to it. No matter what happens, you're going to be okay.
I wish you nothing but the best. :) Meg