Kristenxdollface:
I won't post bible verses, since Poohbear already did that (although if I think of any later, I will add). In my opinion, there is no way to be "overconfident," especially if that confidence is in the Lord. You should always feel blessed by the gifts, talents and blessings that the Lord bestows upon you, and celebrate them. I believe the failure to celebrate God's blessings and grace is a slap in the face to the Lord (I struggled with this for a while, so I understand). Therefore, you are deserving of God's blessings because God is so loving and wonderful.
However, if you have become cocky, that may be another story. You mentioned not being confident in yourself in high school and having low self-esteem. I would warn you to be weary of believing that your newfound cockiness is true confidence. True confidence (in my opinion) is not cockiness, and frankly I have always believed that cockiness and low self-esteem/insecurity are two sides of the same coin. It's like love and sex. Sex may look like love, but sex without love is not love; whereas love can involve sex (I hope this makes sense, I haven't actually had sex yet, but I digress).
Likewise, cocky people appear confident to many, but IMO they are really only masking a deep-seated insecurity within. It appears that you have become cocky, and that your confidence comes from external things (i.e. attention from men and others, compliments) and not from the Lord. While these things can boost self-esteem, true esteem should not be born from these things. Compliments, etc. are like icing on the cake, but not the cake itself. Do you see where I'm going with this? I hope so. You may want to pray about it and ask for God's guidance in how to deal with this. If you are not truly confident within, eventually it will show. You will begin to make decisions based on what others think (i.e. "I can do better--not because this guy isn't good, or even good looking, but b/c others may not approve" that's not necessarily a good thing) and not on what you should do. IMO this can have disastrous results. People choose careers they hate, mates they aren't compatible with, live in houses they can't afford, etc. all b/c they don't have the confidence to make their own decisions. I know that's probably a lot more extreme than your situation, but I think it's worth looking into. Sorry for the long post, and pm me if you want to discuss further!