I have always struggled with getting rid of things. I know my ADHD plays a role in that. I know growing up without a lot also plays a role in that. But ultimately it is something I want to get better at. I have been working my whole adult life to improve at letting things go. Here are a few things that have helped me.
One thing that has really helped me is letting myself keep things. I know that sounds counter intuitive so let me explain. If I am pressuring myself to get rid of everything, I feel out of control and overwhelmed. So by allowing myself the grace to keep things, it makes me feel more in control and ultimately, I get rid of more. For example, getting rid of baby clothes is very hard! So I let myself just pull out the ones I didn't really like or my child didn't wear. Then I put the others away. Next time, I can part with a few more as my memory of them wearing those items fades. I have a few outfits per child that I can picture them in and my heart hurts to let them go. So I don't. I put a small box in the top of the closet just for those. I set myself a limit of one small box for all the baby clothes I'm keeping for all four children. Now, as I find more old clothes in boxes, I feel more free to donate them because I know the special ones are safe.
Declutter in waves! I never make myself go through all of something at once. It's a sure fire way to get overwhelmed and give up! So instead I just choose a few and move on. The next time, I can chose a few more. This works with anything! Baby clothes, shoes, my clothing, art supplies, school supplies, books, etc. It's easier to let something go when you know you are keeping almost all of the rest. But after several phases of this over months, you have decluttered quite a lot.