I have dated guys with acne and never thought twice about it. Though, I do agree with nakedsmurf that girls who have skin problems are probably more accepting of guys with skin problems, so I can't speak from a been-perflectly-clear-my-whole-life point of view. However, my younger sister who was blessed with beautiful skin (I got the shitty genes, of course) dated a guy with pretty bad acne and that did not stop her from eventually marrying him. I know that for us, a guy's confidence, personality, chivalry, humor and thoughtfulness far outweigh any concern of skin issues. I can't stress enough how far confidence can take you! The quickest way to turn a girl off would be crippling insecurity, but there is nothing wrong with having some transparency with the issue if you reach a point in your relationship where you can speak honestly about such concerns. I think most significant others would be surprised by the insecurities and not see the issue as magnified as we often view things about ourselves. And of course, if they're worth your time, they will be loving and supportive about it all. Anyhoo, my two cents!
So when a girl meets a guy who's insecure she goes "Next!" but when she meets a guy who is "confident" she stays interested. And then when later the guy says "You know, I act confident and all but a lot of the time I'm actually quite insecure" she goes "That's ok!". It just seems like a contradiction?
I think everyone is insecure in some respect...but the fact that you overcome your insecurity to behave as confident is much like overcoming fear. Most times when we overcome a fear, it's not because a fear has suddenly gone away....it is living with the emotions of fear to come out on the other side of it. I don't see it as a contradiction, but building strength. Insecurities usually just don't go away even if the "problem" does. For example, I used to weigh about 40 lbs more (at my very short height that is significant)....I've been thin for almost 3 years now and struggled for a long time with the mentality of being overweight. The insecurity remained despite my weight being gone. The act of me behaving confident in my body is what is helping to change my perspective.
And as for OP's question of what is confidence....i think it's being who you are and not acting ashamed of that. It's presenting yourself to the world and not apologizing for your imperfections. It's not looking to the ground all the time or putting yourself down. I think being confident is being aware of your weaknesses, but concentrating on your strengths. If you're the funny guy, or a good listener, or good at sports, or an expert on a particular subject....you let those things shine while not being full of yourself....