has filled your heart

That desire in your heart to do something is not your calling. It is what it is, a personal preference, a desire. You're thinking that all desires are bad, and that's only because you see desire in the lustful sense of the word. It can, in fact, be completely devoid of sexual context. You can desire to please God, for example. No issue with desire in that sense. In and of itself, nothing wrong with desire.

Desire in the heart is only part of having a heart for something. Having a heart for something means not only desiring to do something, but also having a heaviness there that drags your attention and focus away from all other things. It adds significance to it. The significance added to it pulls the desire away from the heart, and places it in the head. It makes that desire an interest. When someone says I have a heart for serving the homeless, they are stating two things: that they have a desire to serve in that area; and that they have a mental interest in doing so.

Where does the Lord fit into this? On one hand, these desires -- not your calling, remember -- are from you. They may or may not be sinful, that's the tricky thing. The Lord can give you a heart for a ministry -- He does that when He touches your soul -- but that should only increase something that was pre-existing. The heart for something must ultimately come from you.

I'm beginning to think I'm splitting hairs here. Hard to tell, but this is going somewhere.