Not to be too uber-critical, but... How do you "accidentally" shoot yourself in the heart? Unless you were holding the gun backwards, it's almost seems impossible to imagine.
Bear with me a bit... Just some supposition...
The character was running away from the door, right? He should be holding the gun naturally, preferably with his finger not on the trigger. He trips, and I suppose he would try to use his arms to break his fall somewhat, and the hands usually end up in the "stop gesture" position. Now, even if he was running with his finger on the trigger, the gun should be pointing ahead/in front of him, not anywhere towards his body. Unless you got off a very unlucky ricochet, was holding his gun some other way, or the gun discharged somewhat, he should be fine, albeit scared, right?
Anyway, keep improving, you'll get better, I'm sure.
EDIT: Hmmm... I suppose if a person doesn't react fast enough, he can fall with his arms bent towards his body. Now, if he did have his finger on the trigger, he really can shoot himself if the gun nozzle just happened to be in the right (wrong?) angle. And you might be able to hit your heart, just not straight on.