Do We Listen To Doubt?

I find I have doubt lingering within me during everything I do in life. I don't think there is one thing I go into without having doubt. Is it normal to have constant doubt? Can we be 100% sure of at least one thing in our lives? I don't know if doubt is a good sign or a constant distraction telling me to stop and get out of whatever it is I'm doing.
If I weigh all the good things against the bad things, the good things out weigh the the bad things. So, should I consider not listening to the negative and continue to think about the positive? Does the negative have to be present in order to keep you on your toes and your head grounded?
If I didn't have doubt I don't think I would try things in the first place. I believe second guessing myself pushes me to try harder and continue to pursue that which is meant to be.
For example, a relationship that has all the makings of something truly special. The excitement, fun, love, and compassion are all in full force. But, the doubt continues to come and go every so often. It is not constant doubt every day or every week, but it does fill my head every so often. This doubt has me questioning the seriousness of the relationship. Like I said, I am never 100% sure about anything I venture head first into. All the good things in the relationship far exceed the doubt which has me constantly thinking about what it is I should do.
Do I fight the doubt or should I give up and give in to the doubt? Everything is incredible except for this doubt that fills up inside me, but then vanishes when time goes on and I stop to think how much this person contributes to the happiness within my life. If one thing doesn't seem right, do I listen to it or do I gather the insurmountable positives and continue to think that his doubt is just a normal human characteristic so I don't get my head lost in the clouds? Everything worth having is worth fighting for.
Do I let go of the best thing that has happened to me because of "doubt"?
I sure hope not.