I tend to keep things for sentimental purposes. How do you keep things from piling up? When do you feel that you really need to let go?

Every once in a while I do a thorough cleaning of my things. If letting go of something makes me sad, I don't let go, if it doesn't make me feel anything or if it makes me happy, I let go. So, instead of "I don't want to/I want to" I tend to focus on my emotional reaction.

When something no longer brings you joy or use. Really ask yourself does this item make me happy to still own or is it just clutter.

I try to think about whether or not I’ll find a use for something later, and try to get rid of most of the things that I won’t need again.

Try to throw out what you don't need, or wear (clothes are easier to part with in my experience). Keep only the things that really matter even from a sentimental point of view. For example I keep gifts from late family members, close friends, statues or pictures of my pets (especially the ones who are deceased).
Gifts from people you were close to in the past and fell out of touch later, you can keep if they are useful. But if you don't use them I suggest you remove them from your house, as they represent a thing from the past, so you should not need them in your life any longer. And if you love looking at it to remember the good old days, you can keep it I guess. But if there is too much, try to sort them and remove some. Think of this: are they really so important if you can't remember these memories without this item?

Well you see, I’m not good at keeping in my emotions so I made an acronym to help me. SWAN, I sing about how I feel, I write what happened, I do some animal therapy, and think about what’s next. That’s what helps me.