Variables are not shared, each instance contains its own variables.
Every time you create a widget, the created widget starts with Self ID = 0 (default value), then event Construct runs on that instance, increasing its value to 1.
To achieve what you want, you should add a counter variable in the blueprint doing the ForLoop. Every loop, create new widget, increase counter by 1, and assign the created widget’s SelfID to the current value of the counter.