Here's a girl come to share her opinion!
As I see it, the reason we put guys in the friendzone is because we're just not attracted to them. But, recently I've noticed something else.
There was a guy who was interested in me. At first, I wasn't interested in him, but he continued being friendly and attentive and I began to like him, thinking he must really like me.
Then, he revealed to us that he, in fact, would pay attention to any girl he thought he had a chance with. So, in fact, he didn't 'like' me, he just thought he could maybe get me into bed. Safe to say I nope'd hard and lost any feelings I had gained for him.
Guys complain when girls are too easy. They complain when we're not easy enough. The 'nice' girls you think should go out with you want to be loved. They want to think they mean more to you than just a committed friend-with-benefits. When you throw too wide a net you'll have trouble bringing it back in, so to speak.
In my opinion, it has nothing to do with confidence. So long as you really like the girl and are attractive (this isn't difficult, just keep clean, dress well and maybe shed a few pounds if there's a need (you can't expect to get an attractive girl if you're not willing to put effort into being attractive yourself)) then she may come to like you over time, if you really are 'nice' and approach her with pure intentions rather than 'hurrhurr I can put my penis in her vagina if I'm nice' then she may come around.
So, let's make a checklist!
1: Do you really like her? If no, find another girl to go after
2: Is there anything physically that would make you unattractive to her (eg excessive sweating) If so, sort it out
3: Does she know you like her? If not, let her know
4: To use a cooking metaphor, put it on to simmer - occasionally check in on her, maybe pay her some attention occasionally, but don't go on too strong or you'll ruin it.
Remember to re-evaluate step 1 occasionally, if you don't like her or you think she's not going to come around it's best to find someone new. And most important to remember; don't get creepy. If you don't know what would entail creepy then you need to go brush up on your social skills.
There's no guarantee it will work, but if you genuinely like her and there's nothing wrong with you then I see no reason she won't like you back eventually. After all, I've done enough generalising in this post to say that guys are all the same.