Well, I can't (and don't want to) speak for others. I do know that I have made more mistakes than the average person and paid for all of them. Here is what I have learned in my life on this subject:

1. Don't decide to tell your fiance just because you feel guilty. IF you tell her, do it because it is the right thing to do. I have seen many families ruined because of one indiscretion and the guilty party thinks confession will fix everything. You are lucky in the sense that you don't have children with your fiance. If you don't tell her, make sure you start living your life the way should, not the way you want.
2. Don't kid yourself - if you tell your fiance, you have probably lost her. She trusted you and you violated that trust. It's pretty hard to rebuild that trust. It can be done but isn't easy.
3. Coupe Girl hit it on the head - do your math and make sure that you COULD be the father. If the math indicates your are the the likely canidate start talking blood tests. Whether you like it or not, you are financial responsible until the child is 18. That is a long time.

I spent most of my career flying highspeed aircraft. I had to think ahead and think thru my actions. You need to do the same. The psych guys have lots of words for what you have done. One problem (there are others) is the fact that males between 16-22 (it varies) impetuously make decisions with out thinking them thru. I have seen it happen many times. You have to figure out how to control it. There also may be other reasons you did this act. It sounds like you are a good person, now act like one!