What advice have you found helpful in the past?

The biggest one is that it is ok to make mistakes! If you must reduce the volume of things you have, it is important to remember that they are things. They might trigger memories, but they are still just things that you assign value based on usefulness or the ability to trigger a memory. But if you lost all of your things in a fire or other unexpected event, you don't lose a part of yourself, just things. This was explained to me by a relative who lost their home in a fire, and while it sounds cliche being able to see them recover was a great learning experience about the importance, or lack thereof, of 'stuff'.

So with that in mind, some things will be more important than others for various reasons. I personally use the business concept of space having value. Storing things costs money. Something taking up space costs money. From a personal perspective, things take up money, space that could be used for other things including easily moving around, and the time and effort to organize/clean/move when you move/etc. Then I determine how much space I want to use for things, knowing it is a goal and not a destination.

Since having things involves multiple rooms and locations, I do the spaces by location. I have a max amount of space in the bedroom, in the kitchen, workshop, etc. The goal is to get the stuff to not overflow from that space into storage. No, I don't generally hit the target but I do massively reduce the overall amount by thinking through how important they really are.

An example of me making a 'mistake' was getting rid of a bunch of RPG books, where I kept two out of the batch for the memories. Later on, I thought about playing again and kicked myself for ditching them. But to be honest, it is very possible to reacquire them and I didn't have to mess around with them for the 20 years between selling them and thinking about playing again. Looked up a pirates PDF and decided it wasn't that much of a loss anyway! Also ditched some furniture when there wasn't enough space, and replacements aren't as nice, but it wouldn't have been worth paying for storage either.

This may not be as helpful if you are having trouble figuring out how much something is worth it to you, so here is a suggested plan that might work as practice.

For things you rarely use anything large, cheap, and easily replaced go first. They are not worth your time and effort to maintain their existence. That may make a big dent in the overall stuff problem.

If you have multiples of the same thing, do you really need that many? Like 5 sets of bed coverings is too many if you don't have plenty of space. Extra blankets can almost always be replaced, and both take up a lot of space and effort to keep clean and organized.

Be honest about clothes. If you haven't worn it for a few years, it doesn't have a specific purpose that is likely in the future and a high cost like good winter gear, try to sort out the easier to replace stuff and keep the most valuable. For me I will never ditch my winter boots even if I moved to a warmer location because a good fitting high quality pair is expensive, but gloves come and go.

If you want memories, try to hold on to small reminders. Maybe a single tub or two. Then focus on what you use regularly, keep that stuff. If you use it rarely, but it is small and expensive and not out of date, keep it but set a maximum size.

I did a massive downsizing when I moved a few years ago and it was liberating. Yeah, some stuff that was kept and some that were gotten rid of were probably the opposite in hindsight, but just having less stuff is a goal unto itself and worth it overall.