How do I tell if it is appropriate for me to seek hospitalization

I've made 2 major suicide attempts in the past, the most recent just 15 months ago. Some days I feel okay & reasonably stable. But other days, I have all I can do to keep from taking another stab at it (no pun intended!)
All of my previous attempts have been spur-of-the-moment affairs. I didn't really have any idea I was going to do it until just before I started swallowing the pills. So I never know from one day to the next if tomorrow will be the day I'll make my next attempt or not.
But I also know that if I can just hold on, another day or group of days is coming when I'll be okay... not great... but not suicidal either. My pdoc never asks me if I need to be in the hospital or if I ever think I do. (He's probably afraid I'll say yes... I doubt I would.)
I've always said that if I'm rational enough to know I'm in trouble then the chances are I won't do anything. But if I'm really in danger, I won't be rational enough to call someone. It's a conundrum!