My fiance is 15 weeks pregnant and its getting harder to deal with her mood swings...help.?

Im 18 years old and my fiance is 19 and is 15 weeks pregnant, We love each other very much but her mood swings are getting hard to deal with. I always become the bad guy even though i feel like ive done so much for us. She always says she is sorry in the end but the things she says aree things i know she doesnt mean but it still hurts anyways. How can i deal with this without fighting back all the time...