Idealising them
Idealising them is rough.
Before and at the tipping point of the breakup, you want to believe they are everything in the world you need to feel whole, safe, and loved.
I, at least, thought they were the person I would marry and end up living a long life with.
But then I realised that I was fighting until the end, and he kept making barriers to us repairing properly. He wanted all of the effort from my end, yet have me accept that he'd resigned so much he didn't even need to speak, smile, apologise, or respond to me until after the weekend.
In reality, if he wanted to be with me, he could and would have been with me. He would have put down his weapons and taken every opportunity I gave them to be close to me again.
Realising this was tough emotionally but still the easiest mathematics I've ever had to do.
If someone wants you enough, there will be no mountain on this Earth that will stop them.