I had the same problem and I agree that you probably have over supply issues or fast letdown. The good news is that things will get better as she gets older. Your body will adjust to her and she will mature enough to be able to handle more milk in her mouth.
You may notice that she doesn't spit up when sleepy? My son spit up a ton while nursing and was awake, but in his sleepy periods he would nurse and not spit up...that is because they are not nursing as strongly, therefore not gettting as much milk.
I would sit up and hold him at my breast with his head on my breast and his body straight down to allow gravity to help the milk down...that helped. So did lying down on the bed with him to help him relax.
I sometimes "pumped" a little out with my hand first to get the initial pressure lessened.
And I reminded myself that a "happy spitter" is okay. If she seems fine when she spits up, don't worry! She's not hungry, just a little gassy!
Hang in, it is frustrating! But it will improve..we now spit up every few days vs EVERY feeding and my son is 16 weeks.
W.