Two things about this sentence.
a) His feelings for Ginny at this point are on a level where he misses his second home less than he misses his ex-girlfriend. He loves her more than anything, and he’s realised that after being forced away from her.
b) This is the only time in the entire series where Ginny is referred to as his girlfriend. The only time, and it is in the past tense, which just breaks my heart.
c) She is referred to as his ex-girlfriend, while later on in the book, when the DA gather before the battle, Cho is referred to as his old girlfriend, which is somewhat less flattering. Note this is from Harry’s point of view. He doesn’t give Cho a second thought after he sees her, but his heart seems to fail upon seeing Ginny.