Keep a small box of your favorite things. It may take days, weeks, or months to wittle it down. For everything else, take photos of your sentimental things for memories, then sell, donate or toss. Sometimes it helps to have an unbiased friend help you weed stuff out. Taking photos is great, because they require almost no storage space and you can print out your favorites.
I've honestly never found the "take a picture" thing to be helpful advice. I'm a very kinesthetic person, so i really like to be able to turn an object over in my hands, feel the weight of it, the texture, the way the light hits it a certain way, all of that. Even if it's tucked away in a box more of the time.
Oh I am just awful about getting rid of things in general. My parents are likely going to retire soon and move, and my mum has asked me what should happen to the piano I played growing up, which is a big RIP when I think about it.
If it's a small thing or something with low value, I usually just keep it and figure "maybe I'll need it one day". Especially if some of the sentimental items are both small and hobby/collector items, it might not even be worth the hassle for the money it generates. Most of the time, though, I really find that if I'm able to pry the item out of my hands and sell it, I never even look back lmfao so I try to keep that thought in mind.