Eureka.

The reason I jump, too fast:
I believe:
If anyone should break my heart, it should be me. Not circumstance nor callous people.

I should stop doing this.
Other people - save one, and he is long gone, now - have been far more careful with my heart than I have ever been with it.
I throw it up into the air and forget to catch it.
I box it when it gets strong enough to soar.
I push it when it wants pulled. Open it when it wants closed.
Reopen healed wounds and leave them bleeding, disgusted.

Only I do this, no one else does it to me.
I really, really should stop.

This isn't really an ahha! moment though. I've known these things for years now.
And still, I do it.
Thornbird, etc.