Have you tried any therapy? Your age is the perfect to start a therapy and trust me you'll notice difference. Consult a speech therapist and ask them to help you build more confidence too. Also speak alot alooooot if you're embarassed to speak at school do it at home or with friends, try different methods like speak slowly, word by word, try to spell out the word like "TRRRAAIIINN" or "TTHHERRRAAPPYY". Goodluck !