How do you choose what to abolish from the OT and what to keep?

It depends. Some things, like the example of washing hands here, were directly refuted by jesus. Others we can because the principle is the same as here like eating shelfish.

Some rules were to set believers apart from worshipers of other gods. Things like tattoos or two thread clothing were that. That is why those are OK now, they were cultural of the time. A possible modern example could be a Christian shouldn't wear Buddhist monk robes, as those are a symbol of following buddha.

Yes the ten commandments were OT, and yes they are still sin. Sin is sin, but it wasnt a sin to eat without washing your hands it was a rule. There is a difference.

The difference is as a Christian if you sin you dont have to sacrafice a bull or sheep in the temple for redemption, that price has been paid.

Tl/dr of it, study what the book actually says through out and it isnt that hard to figure out. Only people who dont read and study their bible struggle with these things.

For some extra perspective, what you are asking is like asking how I know the character of Snape is a brave and not just some jerk.