This animation (which you can find on “Maths Tools – Adding 3-digit numbers”) might help you to understand how regrouping works.
This works for 2-digit numbers, 4-digit numbers and so on…
Practise using this and challenging yourself using paper too, to see if you can do as well as the computer and get them correct.
Now you can also have a go at subtracting with regrouping. Use this animation to see how it works.
This should feel much simpler for you now.