During those rough times, do your best to find some support. Talk to someone, whether that be your spouse, your best friend, clergyman, or other Stepmoms. Keeping things inside will do no good for your relationship with your husband, your stepkids, or yourself. Learn to relax. Don't let the little things bother you.
Life really can be an adventure, just take it one day at a time.
Don't dwell on what others say you do wrong, think about the things you do right, and the good things that you have to offer. It takes someone special to be a Stepmom, and your husband must think you're really special because he chose YOU for that role.