I don't go out and preach because of my desire as much as I go out to fulfill the command of Christ. He said, "Go!" to all of His disciples, and we're to go and witness to Christ by the way we live and by our verbal witness about the death, burial, and resurrection of Christ and about the need to repent and believe. I've never had any doubts about my call.
When Scripture talks about the heart, it's not talking about that life-sustaining muscle. It's talking about our entire inner being. The heart is the seat of our emotions, the seat of decisive action, and the seat of belief (as well as doubt).
