The singer is talking about a girl that he has feelings for, but she has a boyfriend who is a huge jerk and treats her badly, and she puts up with it. The song mentions the girl crying and being put through a bunch of crap. He says she "isn't the same" which means she has gotten so wrapped up in this guy, that she is changing herself for him instead of being herself, like she used to be. The singer says he has to "let her go", so he's saying he has to drop all the feelings he has for her and move on with his life, instead of waiting around for her to ditch her boyfriend.
The singer is talking about a girl that he has feelings for, but she has a boyfriend who is a huge jerk and treats her badly, and she puts up with it. The song mentions the girl crying and being put through a bunch of crap. He says she "isn't the same" which means she has gotten so wrapped up in this guy, that she is changing herself for him instead of being herself, like she used to be. The singer says he has to "let her go", so he's saying he has to drop all the feelings he has for her and move on with his life, instead of waiting around for her to ditch her boyfriend.