I would prefer for my gf to not go on a trip by herself since we just got back together.
What is your reason for not wanting her to go?
Because I'm going to be lonely.
You shouldn't try and control your girlfriend's actions. You should trust her and hope that she has a good time.
What do you mean? I know what I'm doing.
It is not a good idea to try and dictate what your partner does. This may make them feel like their freedom is being restricted. If they continue on feeling like this you are simply going to motivate them to find a relationship where they feel more free.