Exodus 20:12
Honour thy father and thy mother: that thy days may be long upon the land which the LORD thy God giveth thee.
Honor
What the rabbis taught:
The Jewish rabbis have often called this "the hardest commandment."
Why?
For the same reason the OP mentions:
our parents aren't perfect, they're sinners, and some of them might be really really bad, even abusive.
You aren't alone in this:
The conundrum brought up by the OP (how we do honor abusive parents) is NOT a new problem.
It's an old problem.
Being old doesn't make it any easier, but it does mean that MANY people have had the same problem, and it shouldn't be too hard to get some Biblical counsel and advice.
What does honor even mean?
When God tells us to honor our parents, what is He really telling us to do?
The word "honor" is usually defined as "respect", and often with some other attributes added,
like kindness, filial duties, and even obedience.
Sometimes we are able to add all of these other attributes,
but sometimes we CANNOT add all of these other attributes,
and we MUST dial it back to it's simplest form...
and just view it as "respect."
Obviously a small child is EXPECTED to obey a parent in different ways than an adult.
But a child and an adult can both have "respect" for the parent.
Can, and should, an adult obey their parents?
This is where all the trouble really comes in.
A. You clearly should NOT obey your parents if they want you to go against God.
B. You clearly should NOT obey your parents if, because of their sin, or lack of wisdom, they desire something of you that will harm you.
We each belong to God, and have HIS purposes to fulfill.. we therefore have NO RIGHT to allow others to destroy us... we belong to God.
C. There are many acts of kindness, which do not go against God, and which do not harm us, which our parents may desire of us.
These are the kinds of things we need to do.
This is where we just need to show them kindness, and love, and GRACE... even if they are undeserving.
NOTE:
This does NOT mean we should spend undue time around them if they are STILL toxic and abusive.
This would be allowing others to HARM you... and YOU DON'T BELONG TO YOU!
YOU belong to GOD!
You cannot let others HARM something which BELONGS TO GOD!
Conclusion:
A. We owe our parents respect, and grace, inasmuch as it does not go against God, or harm us.
B. We SHOULD NOT allow our parents to harm us... because we belong to God; we are HIS PROPERTY, and we have no right to allow others to harm God's property.
C. Finally, this is an ANCIENT PROBLEM! Much has been written about it, many are familiar with it, and you don't need to go through it alone!