As I struggled with reconciling what I want with what God wants during my own discernment, I’ve come to think of it this way;
God isn’t interested in our happiness so much as our joyfulness. Happiness is based in the physical world and as such is impermanent, temporal. Joy is to be found solely in the Lord and like Him is eternal.
Now there are plenty of things in this world that make me happy and unfortunately a lot of them take me further from God. Some don’t even take me further from Him but do nothing to further my journey towards Him and I know that I must never stop or God forbid, go backwards.
So it seems likely that this young girl asked herself if joining that particular order would bring her joy or merely make her happy and then based her decision on what the Holy Spirit revealed to her.