Cora told Rumpel he was the only man she ever loved, and he seemed to really love her, so, why didn't he change into a human when they kiss ?

I've seen a lot of different theories about this.

One is that Cora didn't see Rumple as being cursed. So when she kissed him, there wasn't anything to be "fixed."
Another is that there is a difference between "love" and "true love." Cora and Rumple did, indeed, love each other, but that doesn't mean they had "true love".
The last one (which ties into the above one) is that Cora didn't really love Rumple, which is why she was able to choose power over him.