A complicated painful break up

Here's the relatively short version- I was in a long distance polyamorous relationship with a fantastic woman(my SO)- we were supportive and loving. I got along great with her spouse and her girlfriend and for 5 months things were amazing, but my other live in girlfriend was not comfortable with her and wanted me committed to just her. So my SO decided to end things between us rather than me choose my live in girlfriend over her. I agreed that I'd support my SO as always, even if I didn't want that.

So why does it still hurt to think about her over a month later?