I need advice on which clutter to keep and what can be thrown away - eg photos, tickets, certificates etc?

How do you decide which ‘memory things’ and past mementoes to keep or throw out when decluttering? Anything you wished you had kept and didn’t or look at now and wonder why you bothered saving it?

I have piles of old letters and cards etc and I wonder if I will really look at these ever again. I have some toys and books I want to show my children oneday. Then there are the endless photos and certificates etc. Do we really need this stuff? I dont want to be rid of anything I will regret but really my swimming ribbon when I was aged 6….....?

Any ideas of how to sort through it all appriciated.