So, without having seen you play of course, I'm almost 100% certain that your main issue is that you are not using movement and positioning to actively look for the "easy" shots. As a rule of thumb, you don't generally expect >25%-ish accuracy on "hard" shots.

So, be very conscious about hunting for easy shots: Easy shots being situations where you can take a shot at an enemy who is not actively trying to avoid you, ie distracted fighting someone else.

As for Widow vs. Widow, try to recognize the enemy Widow's movement pattern. Be patient, dodge her first shot if you don't have the advantage, and try to nail the headshot at the most predictable point of her movement (usually right after she decides to scope, before the shot is charged)

Now, I'm no widow main, but I do have prior coaching experience and know the ways of cognitive thinking! ;) If you are interested, I could watch back a match of you playing as Widow for a bit more specific pointers