I just want to tell you the basics of how to learn those match ups better. Well it helped me a lot and I hope the same goes for you guys!
So the builds you can not beat for instance, 1 base collosus in PvP. Use that build order for like every game. Feel its cons and pros. You slowly learn everything about how to break down that build order when other players use it. Like a guy gets a warp prism and drops your mineral line so you can't move out with your one base collosus army.
Anyways pretty simple use the BO you cannot counter. See how people beat your version of it then you can profit.
I know some of you probably know this but I think many players don't especially in the lower leagues, I really hope I help out for you who are doing bad in those pesky mirror match ups!
GL & HF! ^^