Im highly disappointed in my body, and i need help losing what i gained to feel better about myself.?

so im only 14 but i have always been the little tiny scrawny girl. lately, im not sure why, but ive let myself go and ive gained at least 20 pounds. i just don't feel like i have enough in me to get myself back on shape and back down to a size 0. which i have been since 5th grade. all of my friends say your so skinny blah blah blah but i really don'tt feel comfortablee in my own skin anymore. im not saying im FAT but just not thin and tone like i usually am and i don'tt know how to change it. i think the reason it bothers me the most is because ive always been in shape thin athletic but the past 6 months has been the total oppositet. even my boyfriend of 3 years has noticed iv been a lott lazier lately. i really don'tt want to be a disappointmentt to him. im not sure how to fix this.. please help.