I'm so sorry to hear this.  That is quite a traumatic experience!  You didn't deserve that treatment!

I went through similar things growing up an have suffered other forms of abuse so I can relate to how difficult it is to forgive.  Yet, you know that forgiveness is the only thing that will stop the hurting.

A few things have helped me.

1.  Really picturing myself on the cross like Jesus.  What did He do with the pain and humiliation He suffered?  He offered it up for those who wounded Him.  

Now think about those tennis balls to the head and unite that pain to the crown of thorns and the humiliation Jesus suffered for you. 

 Imagine yourself in Jesus place with the crown of thorns on your head and suffering the jeering and the spit and the slaps that He suffered. Where are the friends who He thought cared about Him? 

Now imagine doing like Him and offer that up for those Roman soldiers.  

Now go back to your scene with the tennis balls. Feel the pain.  Unite it to the pain of the thorns on your scalp with blood running down your face, the jeers, the spit, the slaps, the abandonment and turncoating by friends.

Now offer that up for those girls, for your parents who didn't seem to care and teachers.  Say aloud, "Thank you, Jesus, for allowing me to suffer as you did.  Thank you for the humiliation, the pain, the loneliness, the rejection, the feelings of abandonment.  I offer this up for those girls and for my parents.  Forgive them, Lord, they know not what they do."  

Repeat as necessary.

2.  Think of times you have really wounded someone with your sins.  Now think about how you really didn't have full understanding of how badly your words or actions would hurt them.  You weren't in the right mind or you would have never done it.  Confess that sin to Jesus and think about how forgiving He has been to you.  If He can so readily forgive you for those sins and others sins you've committed, surely you can forgive those girls.

3.  Take out a sheet of paper and number it from 1 to 20.  Now with just a few words on each line write a short phrase.  Each phrase describes something good that came out of that experience you had.
What?  How?  Oh yes.  You'll see once you get started. 
1.  Taught me empathy
2.  Made me a better parent
3.  Gratitude for forgiveness of wounds I caused Jesus
4.  Made me tougher
5.  so on and so on till you reach 20!

Nicely done.

It still comes back from time to time and I have to rinse and repeat but the pain gets less and less and at least I can make use of it and hopefully help to save my soul and the souls of those who hurt me.

I'm sure it must. It is a most useful and compassionate post. I will pray for you as you try to implement it. 

I can say this, schools, teachers are not allowed to discipline.  they can not tell any student to stop doing something, for example what you went through. This is in the USA. In the year 1995, no disciplining, or teachers could be sued. Sounds like the same from where you are from. UK?

I worked on the school buses as an assistant, and that is how I found out about no disciplining.  

True, parents are not perfect.  True, you are older now and can be yourself.  I have a neice in Scotland with 6 daughters home schooling. My niece is married to Stewart, a composer, teacher of music.  Both came from parents like yours.  Both have asked us to adopt them as daughter and son. OK! They had no catholicism and in 20 years are now traditional.

I find it best to say to myself, forgiving is divine.  You take God with you, never suffer alone. Christ had an angel with him in the Garden. Take your hurts to the foot of the cross.  Remember what is truth. If I feel yucky feelings, I take Mary with me to the Cross. If I think the devil is pestering me, I remind him of truth, that reminds me of truth.  Truth is I am old enough to learn and grow, and Mary is with me.  Rosary is very important.  I was told my a priest to say 3 rosaries a day for the hurts I have.  I made no promise, but 3 rosaries a day is my goal.

Truth is we all have a cross to bear.  Truth is there are no cowards in heaven. Truth is we all must suffer and how we manage it is the solution.  Solution is the Precious Blood of Christ is in all sacraments and sacramentals, rosary.  The demons hate it and you do it!  So, what can I do, or you do?  You know the remedy!

I find it very easy to forgive ... with this in mind.  I realize the enormity of my own sins and that if God can forgive me, how can I not forgive others?  It's just like the parable from Our Lord that was forgiven a huge debt but then went to throttle his fellow servants regarding the pittance they owed him.  If I failed to forgive anyone, then I would deserve to be hurled into the deepest corner of hell.

Also, I think it's important to distinguish between forgiveness and not having some sore feelings over it.  There are some people who have wronged me that to this day the very sight of them bothers me on an emotional level.  I just don't like them.  But that's different than not forgiving them.  So just ask God not to hold the misdeeds of your parents against them, even if the memory of it still causes you pain.

Put another way, there's a huge difference between not wanting them punished and not having ill feelings toward them ... the two of which can be confused.  Do you want your parents punished for your sins?  If not, then ask God to forgive them.  That is in fact forgiveness.  That doesn't mean you feel all "lovey dovey" toward them in this regard and it still doesn't cause ill feelings.  We Catholics distinguish between feelings and the intellect/will.  And this is important in many areas, but especially for forgiveness.  If some guy tortured me in a prison for years, I can ask God not to punish him, but that doesn't mean that every time I see the guy I don't have a negative emotional reaction and that his mere presence doesn't cause me an emotional revulsion.

Here is a most beautiful sad story of the forgiveness of a couple for a man who killed their first three of their six children.

They did that from the first moment of their loss. Attached to the article is a 54 min video of which I have only watched 7 minutes. I hope it helps you.

https://7news.com.au/news/nsw/parents-forgive-the-man-responsible-for-killing-their-children-in-tragic-oatlands-crash-c-1290233

If your parents ask you to forgive, you should forgive rhem. If they don't ask, you don't have to forgive them.

Forgiveness is an act of the will, not of the emotions, or of the intellect.  Don’t mistake your thoughts and feelings for the choice to forgive.  Start by asking the Holy Ghost to give you the  will to forgive.  And actually pray it, aloud, in the hearing of your guardian angel, the Saints, and yes, in the hearing of the Devil and his minions. That way, when tempted to nurse your anger and hurt, or to review your grievances, you can rightfully send the demons to the Foot of the Cross.  Call, also, on Mary. Our Heavenly Mother not only listens with complete sympathy, but sends the Enemy packing.  Satan won’t hang around because Mary fills him with terror.