Came up with this last night (first two lines just popped into my head, so I ran with it.  No real person in mind, though.

Heart On Lock

How does it feel to know

You got my heart on lock

You got me in a vice

And can do what you want

I don't know how I got

To this point in time

All's I know is that I

Just can't stop thinking

What if shit worked out right?

What if we went through with

What we were planning to?

Would this have turned out right?

I don't know the answers

I don't know what to say

All's I know is that now

You got my heart on lock

But despite all of that

Despite how I'm feeling

I'm also quite aware

We just wouldn't work out

But you still got to know

You've got my heart on lock.

We go our separate ways

But I won't forget you