I don't really have the balls to talk to the girl I have a crush on when she's talking to other girls.
That's pretty reasonable. When you're gone they will began discussing you in a way possibly more laddish than you have heard from lads.
But then I see some guys just casually stroll up to them and my self-esteem takes another hit :/
I've found unlimited arrogance to be the best defense against self-esteem issues. Someone else doing it better then you? Nonsense! Clearly that fool is relying on luck and and circumstance, blundering his way to success that is artificially pleasant but hollow inside; while you have a plan subliminally appearing in your head that will spring itself upon some lucky girl, nay, the perfect girl (who will of course love you unconditionally). Alternatively, they should stop enforcing gender stereotypes and ask
you out; anyone that won't clearly wouldn't be up to your standards. See? As your arrogance increases the number of situations that cannot be explained away approaches zero.
Or perhaps, you should stop judging yourself by what some other, random person of no import is doing. Clearly they don't affect you, their reasoning is irrelevant to your situation, and you have just heard from people here that it is reasonable to think that a girl being surrounded by friend girls is not an auspicious time to ask; why let your preconceived notions of what the "right" way or what a "man" would do get in the way!? Remember that fools rush in where angels fear to tread, and a game of chicken is for those less self-assured.
Ok, while one paragraph was highly unhelpful, there was some genuine advice in the second one.