Epistemic status: provisional.
I realize that I've been adding more to the idea of hardening, or finding that I have to clarify issues with it. The topic is important, so that's justified. But I wish that I had figured all of this out before I started writing about it. Normally, a blog is a good medium for working on thoughts. But if you say something that concerns risk, it seems good to have a finished product before you say things that might mislead or alienate people, for being incomplete presentations of a concept. However, since I have so few readers (as far as I know), perhaps I have time in which to develop what I have to say.
What would it be like to harden yourself? How would it feel on the inside? Well, you have to reject God and cease to listen to his voice. So you would be conscious of rejecting God. You would make the decision irrevocably. You would reject with your whole being. Buber said evil is never done in a whole-souled way, and I can see something to that here. You have to violate a part of yourself to make this decision. But you can do that, and if you harden, then that is what you do.
What would it feel like to be in a state of being hardened? I'm not sure. One possibility is that you just wouldn't care that you were hardened, or what God cared about. This seems like a likely phenomenology of being hardened. Another might be that you don't care about those things but pretend to yourself that you do care, knowing that you pretend. I'm less sure that that kind of thing happens, but if it did, I think it could be a case of hardening, because the self-deception could be deliberate enough and knowing enough.
Much of what I am concerned about when I think about myself hardening is getting into ease. Ease can lead to a kind of nihilism on a deep level. But I haven't hardened yet, if I am indifferent. I may be in a state of danger, but I haven't made the fatal step.
Is it possible to have made that decision and then forgotten you did so? If you did so, you wouldn't care about the question. If you feel that you don't care, but you choose to turn to God for salvation, then you weren't irrevocably hardened.
I don't think very many people have consciously and knowingly rejected God in this life in an irrevocable way. The next life (the Millennium), will last a long, but finite time, and toward the end of it is when I would expect to see the most hardenings actually happen. What we do now, at any point along the way, can affect how close we get to that possibility. Whatever sins or idols we love more than God, that we hold onto, are things that could lead us into hardening someday. But, the door is not shut. Unless you know that the door is shut, since you decided deliberately and irrevocably to shut it, and don't care that it is shut.