What should I do, if someone I love doesn't feel the same about me and is, hopelessly in love with another girl who doesn't even like him back?

You can either keep putting yourself through this pain and wait to see if anything happens with your relationship or you can try to move on and invest your time and heart into someone who also sees you as someone worth fighting for. It’s really up to you, love. The only advice I can give you is rather vague because I only ever seen two outcomes with unrequited love.