Yep, been there, done that. A few things that helped:
1. Weekly review. Top priority. If you're doing your weekly review, you'll have already thought those insignificant tasks through to confirm that it's insignificant.
2. Regular brain dumps. That might even be every night to start with. As you then process those brain dumps, you'll gradually reassure yourself that everything really IS in the system (or they will be by the time become sick of brain dumps!)
3. Daily review - just a quick skim of action lists to remind yourself that everything's under control.
4. Make yourself a quick note about why you decided something was insignificant. That way you don't have to repeatedly think it through.