It's all about limits. Think about when you're teaching kids and they have to keep different subjects in different notebooks, or they have to keep a reasonable amount of stuff in their lockers—it's no different with your garage.
You need to establish a clear limit on what you're okay with keeping. Two shelves for textbooks, three bins for holiday decorations, an album and memento box for each of your overseas trips. Then you need to consistently work through the items in the garage for a half hour every day, working toward cutting items down to the limits you set.
Doing nothing is the worst thing you can do. Set some limits and understand that small steps, consistently applied, will yield results.