How to start unconditionally reinforcing your boundaries

I need to start unconditionally reinforcing my boundaries with my pwBPD. To make a long story short, we are technically broken up but it doesn't feel that way, and ultimately, to get any positive outcome - whether that's in the relationship or not - I need to unconditionally reinforce my boundaries. I have been trying to do this and failing for months, even before we broke up.

The problem is it is extremely difficult to rewrite what your brain has been trained to default to for years. Sometimes I don't even realise I'm letting my boundaries be crossed, or compromising on them to avoid triggering my pwBPD, until after the fact.

Anyone have any words of advice?