I don't think you will be able to get your parents to understand you. As you've described it, they are cruel to you for their own internal reasons, rather than because of anything you've actually done.

As is always the case when you have a behavior (be cruel to daughter) with a fake reason (because she deserves it), and a real reason (because it makes me feel big/proves I'm not old/lets me get back at my possibly imaginary tormentors/vents my sadism/whatever) you have to be careful to model them correctly, in order to accurately predict their response.

Most likely, they genuinely believe in their fake reason. They don't see themselves as constantly tearing you down to build them up, but as fairly, judiciously assessing each instance, and acting appropriately. From the outside you can see that their response will always be condemnation, but they believe that each instance is provoked.

If you address the fake reason (Parents, it is unreasonable to be angry at me for being late. I was not actually late.), they'll just parry meta. (Who are you to tell us what's reasonable? Why are you making such a big deal about this? You just want an excuse for your tardiness.) That is, if they are in the wrong they'll just change the issue to you pointing it out.

On the other hand, if you address the real problems (Parents, you must stop berating me in order to feel better about yourselves), they'll deny that that's what they are doing. (What are you talking about? What kind of child would accuse their parents of being evil? Don't you know how much we love you?)

I think you won't be able to fix things via a letter/lecture as you are contemplating. Any letter you send them, any conversation you have, will be blocked in one of these two ways. If you point out the trend, they'll deny. If you discuss any specific instance of the fact that they don't see you as a person (say them stealing your mail), they'll criticize you for noticing their wrongdoing, rather than respond to your accusation.

So if explaining what they've done and fixing it is out, what's left? The following is without knowledge of your personal circumstances. Perhaps one or more of these options are closed to you. These are the three basic paths I see.

1. You can cut ties with them entirely. They will, of course, blame you for it, but they can't make you experience their condemnation if they can't communicate with you.
2. You can cease to hope for better from them. Accept their judgement and scorn and render empty apologies. Lots of people keep pets, on which they lavish love and affection and receive animal lack of understanding in return. Yours are just people shaped instead of fish shaped.
3. Continue in your current vein, that is, continue to communicate with them and continue to expect them to behave better. Perhaps they will change.

Only you can say which of these three options is best (or of course, I could have missed something and a 4th option may be superior). Whatever the case, good luck.