Hello Nevayda!
...well I suppose I have a question for you! Are you in Love or dependent on your girlfriend? ...and is there any doubt that she is in love with you? I have heard a lot of men say they love their wifes, but rarely to I hear a man say I'm "In Love with my wife/Girlfriend"
To me there is a bit of a difference...well actually a big difference! I suppose a wife and husband can survive a relationship with one partner suffering from depression..if they really take all of the vows seriously. I see way too many a young couple skip over the "In sickness and health" part of the vows.
I do think you need some counseling first, and it might be a good idea to see if she can be involved with it as well. If you are really worried about cheating...then there is some doubt in the relationship...and that has to be fixed first..before moving on into a possible marriage.
I don't post here in the depression forum often, but happen to come across this post, and wanted to comment on it.
Good luck, and I hope you can work out your fears in the relationship.
SE