Hm...I honestly don't know if such a thing as partial depression exists. Always thought it was either you have some kind of depression or you don't have it at all. Although it may be conditional depression. Maybe you're just sad because you're parting from your girlfriend. Usually happens with most couples.

Are there any other times where you're depressed like this? Or is this the ONLY time?

It happens at other times. I does happen once we are apart, but it also happens other times too.

Maybe it is depression then. :/ But what about these other times? What happens then?

I think you should have your self looked at by a professional. They know what they're doing and can probably figure it out. It sounds like either Depression or Bipolar which are psychologically similar in so many ways.

Since you are my friend and I know you pretty well. I am pretty sure you have Depression.
It affects different people in many different ways. As does every mental disorder. I think your girlfriend only sees your happy side, or maybe doesn't want to admit it.
Before my depression was as bad as it is now. I would always feel bad that my Ex had Depression because I felt that I should be able to make him happy and not depressed. However, now I know it doesn't work that way. The only way to get yourself out of it is to realize you are Depressed and get professional help.
Even if your psychiatrist or MD diagnoses you without Depression you could have something else like GAD (General Anxiety Disorder).
I would just go. Its for your health. And I don't think this is the case, but if she says she doesn't want you to go or tells you not to. Tell her you NEED to. And tell her if she loves you then she'll be fine with it.
She can't be your final decision with your own health. You know you need to see a doctor. If the reason you don't go is because she says you don't have depression.
You were sad before her, and happy with her but not ALL the time. I'm that way with Pat. Plus my meds are working.
So if you get meds and therapy, plus you can hang out with your girlfriend... it will be amazing for you :)

Oh, and there is no such thing as partial depression. You either have it or don't. Its called Major depressive disorder, a.k.a. clinical or major depression.
Now there are 5 subtypes: Atypical, Catonic, Melancholic, and Seasonal Affective Disorder (SAD). There is also Postpartum.
They all differ by symptoms, but there is no "Partial" depression.

Thanks for all the help guys. I am looking into getting therapy. I just have a problem with asking for help. I know I need to, but I can't do it myself and I can't ask for help. It scares me and makes me feel weak. I don't know what exactly I'm going to do though. We'll just have to wait and see I guess.....

You know, I just thought of an idea for you.
Instead of talking to your mom and telling her that you want to set up a doctor's appointment, write or type her a note.
Tell her how you feel and that you know you need help.
Then she will be able to help you set up an appointment, go with you, and help you through it so you don't have to do it alone.
I know at my first appointment, my dad came with me and talked a little. Then the doctor asked him to leave to get more stuff I couldn't say in front of my dad.
You'll feel so much better once you get it over with and maybe prescribed meds and into therapy (: