Initially, having a difficult child helped me to be more empathetic towards other difficult children and their parents. It also helped me do better research, brush on my vocabulary and stand up against the most intimidating presence in my child's life - the public school district. Having a difficult child helped me strengthen myself against those who criticized me for parenting her 'differently' and doing what I believed was right even if it went against others' well intentioned advice.
Throughout the years of advocating both for difficult child and then with difficult child in regards to school and her social skills, it's been enlightening in many ways, most of which was in regards to the difficulty she had believing in herself and making healthy choices...always the battle.
I realized that as a parent you cannot just say, "Do it like this" and expect them to follow suit. Even when you point out the consequences of their poor actions, they still may not follow suit.
So, I guess, the best things I learned from having a difficult child has been
a) empathy and
b) let go a alittle - hold on loosely except when it's a matter of life/death.
Today, I believe what I've learned most is that once a savvy teenager realizes that you believe she has a disability or disadvantage over her peers, she will use it to get her way, manipulate and control you and the family as a whole...in which case, as a parent you develop certain survival skills that some other parents never have to contend with and the first is detachment.
Out of love, you learn to let them go and fall flat on their little chubby difficult child faces. That takes massive amounts of strength and I'm not completely there yet, though I am a work in progress. I marvel at the parents here who have mastered detachment.