You are right. The physical wounds of child beating will heal themselves in time. It is the emotional and spiritual scars that can last a lifetime. Indeed, it is not the severity of the beatings that matter as much as the atmosphere in which they are administered. If a child lives in an environment where they can never measure up, or the rules change all the time, the scars on their phsyches can be very severe.
I do believe that a modest amount of physical discipline is necessary for some children. However, it should never leave marks of any kind, and it should be used as only a last resort and in a consistent manner. The parent should also remember that not every child needs to be straightened out thus. Some children are very reasonable and do not require any kind of physical chastisement. Just a word or a look will often suffice.
The anger is also normal and OK. Yes, both God and Jesus have been angry. It is called righteous anger. The fact that your parents took away your feeling of safety and unconditional love at such a tender age is surely worthy of righteous anger. They will have to answer for it on the last day. The only time when anger becomes a sin is if you sin because of it. There is a verse in the bible that states that one should be angry and not sin.
The reason that the bible states that you should not let your anger go past the sunset is that when you are lying in bed angry, you tend to get very dark thoughts. Thoughts of rage and hate. You become more vulnerable to the suggestions of the Devil. If you are feeling angry, you need to take it to God before you lay down to sleep.
Forgiveness doesn't come all at once. You will find that over the years you will become angry over and over again about what your parents put you through. That is perfectly normal, especially since the hurt takes a long time to fade. Forgiveness is a choice that you have to make, not an emotion. Sometimes you have to forgive over and over again, because the hurt continues to well up. That's OK. God understands and knows how badly you hurt. He will comfort you if you ask Him.
There are a lot of ways to deal with the anger when it comes. One way that works for me is to pray for the person with whom I am angry. I find it very hard to be angry when I am interceding for them--and the Lord knows that they need all the prayer they can get. Sometimes the only prayer I can muster is something like "I pray that they don't drop dead just now." God has a sense of humor, too.

If you keep trying, you will find it easier to ask for God's grace for them, as you begin to see just how deficient in it they truly were when taking care of you.
I hope this helps. May God pour out blessings and peace upon you and take care of you in ways you never imagined possible.