Well that is a key skill to learn and it is hard.
The math isn't wickedly difficult but the situations are complex enough that doing the math in your head in a few seconds at a poker table is beyond the skills of most players. My alternative is to play with pokerstove or similar software and learn some rules of thumb. You don't need precision, a rough guess will get you through most situations.
I find players have a mental block with the concept of range. They want to assign a specific hand to the range not a cloud hands to represent the uncertainty of what villain is doing. This process benefits Heroes all the time. I can't tell you how many times people assume my raises mean I have exactly AK or AQ and play their hands accordingly. They even will say as much at the table.
These are not "book" skills. Ranging and estimating EV are "homework" skills, things you learn through practice and repetition at games and in after action review of your game. Best thing you can do is practice guessing what hands are in play when you aren't in the hand - ranging - every hand. Yes, that is a lot of work! Typically players do this about once an hour or less for those big exciting hands with huge pots and big bets. Don't be that player - build your poker muscles by exercising them often.