Subnets are a b****
I'm pretty good at maths but it still confused the hell outta me! There are lots of videos on youtube just keep reading and repeating it will stick eventually.
But tbh I wouldn't worry about it too much. If you are not in the business of actually setting up new networks you will rarely need to work out subnets. Working 1st line support I'd worry much more about knowing how to "switch it off and back on again"