Fears about becoming a mother

I'm 31, married for 2 years and we are both employed and talking about trying for a baby this year. However, I keep going back and forth about whether or not I could handle being a mom. I've always wanted kids, so it only makes sense to have them! But sometimes I just don't know if I'm able to handle all the responsibility and stress. I babysit my niece and nephews (ages 2, 3, and 5) frequently and sometimes I'm ready to run out the door screaming cause they rarely listen to me, they are loud, and they make such messes! feel so selfish. I want kids but I'm also really loving the quiet! I also worry about the cost.  I can't imagine never having children though. Is this normal? Have you had similar fears? How did you handle them?