My Confidence and Self Esteem problem?

Okay so a couple years back I was a carefree person. But I started noticing that I might not be so good-looking and so I started to try losing weight and stuff. I'm a freshman in highschool now btw. People compliment me that I look attractive but I still have a self-esteem and confidence problem. It seems like most of my self-esteem or confidence comes from when I look in the mirror and if I think that I look good then I feel okay. If for some reason I dont like the way I look I might feel depressed or feel down or something. I really want to be more carefree. I need more confidence! I've become shy and quiet and probably way too self-conscious. To top it all off, I have a huge crush on this girl and I can't seem to get the confidence to talk to her because of my shyness. Is there anything that I can do to boost my energy level, get rid of my sometimes depression, be more carefree, and have higher self-esteem. I kinda feel insignificant if people around me (usually my friends) are all talkative and seem to have no problem talking to others, while I stay more quiet. I feel kinda like I lost my sense of who I am, that may sound corny, but I really do feel like it. Someone help please?