What is one supposed to do when one is feeling absolutely awful, in need of help, and somewhat suicidal but one is aware one most likely won't go through with it?

In my experience, there are two pieces to this.

First, find friends who actually give a crap about me, who want me to be happy and fulfilled, who are willing to listen to me talk if I want to talk, who care what happens to me. (Professional therapists can also serve this role, if nobody else is available. That's not to say it's the only role they serve, merely to say that they can serve this role.) This is the most valuable piece; without it not much else works.

Second, simply describe my current state. For example: "I feel awful. I've been thinking about killing myself, and though I don't expect I'll ever actually do it, I suspect that the suicidal ideation is itself a bad sign. I don't seem to enjoy anything, I'm either sad or indifferent most of the time, despite there not really being anything in particular to be sad about, and I can't imagine it ever getting any better than this. I need help and I don't know how to seek it out, and I'm afraid that even admitting to this will cause people to think poorly of me."

Of course, it's all easier said than done.