Got big company on-site interview, no CS/EE bachelors, should I feel good?

Since I don't get any real feedback on my performance unless they hire me, and they hire maybe 1 in 4 of those who make it to the onsite, that means I am left to ponder how good of a programmer I actually am if they don't.

When I am feeling good, I feel like I am, maybe, a programmer.

It's not like getting an on site is something I can put on my resume; that would be horribly crass. But I figured I could ask here and maybe, hopefully, not be mocked horribly for feeling insecure.

I love programming, and I want to be good. I'd like to feel good about the interview. Like I am making progress, and shouldn't give up and try to work doing something else.

At the same time, I can't help but feel like I might end up being walked out halfway or something because I say quick-select is O(N) instead of O(log N), or I haven't memorized Boyer-Moore or Fisher-Yates or some clever algorithm, or I just freeze. Maybe they are only interviewing me because I'm in driving distance from Mountain View. I have other people tell me that I am a good coder, but I find it hard to believe them. If I was good, I'd have a full time job, right?

Sometimes I feel really confident, like just after I get payed by someone for freelance dev work. This isn't one of those times.

I guess I want to know, does getting an on site make me good enough, for some value of good enough? Feel free to be brutal, or just ignore this. Or wish me luck, I guess.