Believe me, I know what you mean, my wife has a relationship with a man who lives far from us; they of course spend a lot of time messaging on Skype and such. It used to drive me crazy with jealousy. Everytime I'd leave the house, I'd be worried about them messaging each other. It took me a long time to get comfortable with it, and my wife was amazingly patient with me, but as I became more comfortable, I made it clear that I had a need to feel like I came first for her. When I am at home, I want to have her attention and affection focused on me. She has been great about this, she has moved her computer from the office to the bedroom, so that it isn't so secret what she is up to. When she wants to message him when I am at home she asks me if that would be okay. Not that I am her keeper or anything, just that she respects my need to feel like our family comes first. I never tell her no, but it is good to know that she is respecting my boundaries.
I get the feeling that your needs may not be being met, and your girlfriend may not be respecting your boundaries; probably because she doesn't know what your needs and boundaries are. Spell it out to her now. Let her know that you feel slighted when she doesn't respond to your messages, be flexible (like you have been) but also be firm. Your needs and boundaries are worthy of being respected. Never forget that!
There can be no failure to a man who has not lost his courage, his character, his self respect or his self-confidence. He is still a King - Orison Swett Marden