Well, this sounds like a combination of a couple of things. First, I think you may be right about the depression bit. The self-deprecating thoughts and being easily upset are indicative of that. However, I think that the second-guessing and going back and forth on your thoughts/feelings sounds much more like an anxiety disorder. For example, it's not uncommon in an anxiety disorder like Obsessive Compulsive Disorder for someone to obsess about something they thought or said until they have the compulsion to revisit the discussion - thus, stressing the relationship even more. They know that the thoughts aren't rational (just like you mentioned knowing that the thoughts about your current g.f. not loving you don't match with her loving behavior), but they find that they can't stop their brains from going there. Even worse, they act out on a compulsion to discuss it or confront the other person because their anxiety becomes so high.
This form of OCD often goes unrecognized because clinicians are looking for the typical presentation of repeated washing or checking, etc. However, relational OCD can be just as damaging and difficult to cope with. Luckily, treatment with an anti-depressant like Prozac or Paxil should wipe out both the OCD and the resulting depression.
Please let me know if you have any more questions!
Thanks and good luck to you!
Hey Miranda -
Thanks for the feedback. :) If you go in to see someone about it, I would mention that you think that it could be OCD if they aren't picking up on it. I really think that might be what's going on, and, as I said, a lot of clinicians don't pick up on it if you aren't performing physical rituals.
Either way, I think that an SSRI would help a lot and that you'd get a good result.