You sound in a state of panic. Whenever your girlfriend is away from you, you are experiencing what I think is abandonment anxiety--that is you're afraid that she doesn't love you anymore and that she is going to drop you.
This type of terror usually stems from a wound sustained during early childhood, usually when the child is eighteen to twenty four months old. During this phase, the child begins to separate from mom, but he/she needs to return to mom often for reassurance that she is still there and still loves him or her.
If the mother has unresolved separation issues from her own childhood she may either force her child not to separate, which makes the child anxious and/or she may punish the child for separating and ignore him when he tries to return to her for reassurances, which also makes the child fearful to stray from her.
The terror you feel when you are separated from your girlfriend sounds to me like a replay of an earlier experience that didn't go right. Try to recall any and all memories of your early life. See if you can make the link between then and now.
Then study how this girl differs from your mom. Is she really pulling away from you the way your mom did? If you can say that this girl is different from mom, then just seeing reality may help you to get perspective on this situation.
If you still feel stuck, then find yourself a therapist who specialized in modern psychoanalysis. This type of therapy is especially geared for helping people to heal emotional problems that stem from very early in one's life.