It may be a career move or a new educational pursuit. Or a change in your living situation. Or a change in your church affiliation, or a new venture in using your gifts within your church. You may have approached this decision carefully and prayerfully and have good reason to believe that God is prompting you to go ahead. At the same time you're dogged with doubts and fears and a general uneasiness about making any change at all.
Second thoughts are normal.
Yes, you may look with envy on friends who leap into marriage with perfect confidence that they have found God's choice, or on those who make career changes with surreal assurance that they're following God's will. Remember, though, that you are constructed differently psychologically than they are. You may even be a deeper thinker. And they may be ignoring misgivings which will come out later in more damaging ways. Be thankful that you recognize your feelings and are not repressing them.