How to be more confident? (Girls Only Please)?

Hey, my name's Amy and I'm 12 (6th grade). I have trouble being confident with stuff I guess you could say.... In my school we need to change into gym clothes for PE (I don't like wearing undershirts by the way) So I always need to like do this weird shirt thing when some girls just completely strip off their shirts. I'm asking how do they get the confidence to be half nude in front of 10-20 other girls?! I'm also going to camp for three weeks this summer and it's a sleep away camp... Which means I have to change in front of like 5 other girls. I wear a bra by the way so I shouldn't be worried, but what about when I'm putting on my bra?! We need to walk a bit to get to the main bathroom (we have the portable potty ones) and I am NOT changing in that It's way to small! So, my question is how do I get more confident with this. I also won't even change in front of my mom. Which I know is really weird because she's my mom for crying out loud! But could you please give me some advice, tips ,and maybe personal stories if this has happened to you or if it is happening to you right now. Thanks in advance, ~Amy