Wow K., you are in a tough place. Staying in the marriage is definitely better, the most recent article I read was on the cover of Time Magazine about 4 weeks ago. However, you will find articles to support both sides of the issue.
It sounds like you are probably going to end up leaving, but here are my Words of Wisdom after 20 years of child-rearing, a divorce from the father of my oldest child, and a lot of intimate involvement with many other children:
Divorce is scarring to children. Unless you are in an abusive relationship or one in which the two of you fight constantly, it is better to stay. You know how it would break your daughter's heart to lose her daddy, and ever since my first child was born I knew his happiness was more important to me than my own, and I would never do anything to take his daddy away from him (my first husband chose to divorce, and I could not stop it.)
Do not look to your husband for your happiness. Happiness is found in your friends and the causes you devote yourself to. I love my husband, but do not consider him my "best" friend, and that is ok. I have plenty of other friends to fill that role. We don't have a ton of things in common, but we have enough to unite us. So it is possible to be happy with someone who is dissimilar to yourself.
Marrying young definitely makes it all harder, but time does fly.
And Cheryl, below, said everything else that I did not say.
And in response to Rae's response below, I must say that what you will do is teach your daughter to really get to know the person she is marrying beforehand, and to NOT marry young. We can teach our children from our mistakes, not force them to repeat them. I'm sorry Rae -- it is really sad to think that a woman's happiness is dependent upon her husband. "Misery?" Unless K.'s husband is a bad guy, life has too much to offer that can make a person happy.