I do not think that you are clueless, or, an idiot.

You are making the conclusion you are making after collecting data - observing the subject's interactions with other young women. Before you collected the data, your made a wrong judgment due to insufficient data. When more data came flowing in, you adjusted your conclusion based on that. So far you sound very reasonable and certainly not an idiot.

I think you are not fully in touch with the root cause of your feeling so embarrassed now.

It seems that one thing is definitely at play and, additionally, another thing might be at play.

Let us say that when the subject sent the signals he sent, you estimated that his opinion of you and his interest in you was at 6 out of 10. Then, when you noticed the wide net casting method of his, you realized that you were wrong. You then estimated his interest in you at 2 out of 10. So it as if you had been downgraded. People do not like being downgraded and your reaction is perfectly human.

That additional thing seems to be that you let the subject know that you estimated his interest in you at 6 out of 10, and now that you realize this is not true, you feel as if he were laughing at your wishful thinking. And that gives you a very uncomfortable feeling (and would give that feeling to most people in your shoes) which you vaguely describe as feeling like an idiot. You do not feel like an idiot because of the failure to discern that you weren't special to him; you feel exposed and vulnerable as if the subject caught you with false hopes.

At least that is what it seems to be from just reading what you have written. I assure you that the vast majority of people at some point or another would be in your shoes and would feel equally queasy.