When people say to be a "challenge",what they mean is not to be easy.
You NEVER,EVER,want her to think that you are completely won over.
I have a good analogy for this. Let's say that you have two teams getting ready to play a basketball game against each other. One team is already on the court,they're pumped up,ready to go,and waiting for the other team to come out on the court. Then the other team comes out,sees the first team,and they are so afraid,so intimidated,that they just give up. They forfeit. So,now that they have forfeited,the other team is declared the winner. Now,although the first team "technically" has won,and they are the winners,do you really want to win like that? I mean,you have the trophy,you have the title and everything,but it required no effort on your part. There was no "challenge",no thrill of victory,no highs and lows,no last second heroics. There was no sense of knowing you had this strong opponent trying to defeat you,but you gave it your all,you climbed the mountain,and you were the best between the two of you. You may have the trophy,but all the things that make the trophy so meaningful were not there because the other team just gave up without a fight. If you act like you are completely won over,like the only thing she has to do to win you over is simply exist,then this is what you do to the girl. If you are completely won over,there is no thrill for her,no challenge,no sense of achievement. She may have you (the trophy),but all the fire,the passion,the drive,the emotions,that should come with winning the trophy won't be there. And then what happens? She'll get bored and start looking around for someone else who can generate feelings and emotions in her. So keep her on her toes,switch up routines,be unpredictable.