First, understand that many people find it difficult to let go of things. That's okay; it doesn't make you lazy or bad. Most people feel that way about some categories of things.
Second, there are many reasons for holding onto things. The better you know yourself and your reasons, the more you can address the underlying needs in a way that make it possible for you to condense things effectively without hurting yourself in the process. Frex, you might have memories of not having things when you needed them, or you might have little in the way of resources now, or you might have been raised by people with those traits.
Some techniques and principles that I have found helpful include ...
* Reduction may be made by throwing things away, giving them away, or selling them. If something is broken, it should usually be thrown away. If you can't bear parting with it, try to think of a craft or upcycle project that would make it useful again. I often use old coffee cups as pen holders, where a chip or missing handle won't hurt anything. If something is useful but not very impressive, give it to a friend who needs one or donate it to a thrift store. Nice things that are just surplus may be put into a yardsale or taken to a specialty retailer such as a used book store. Any money you make from selling discards should either be used as a reward to motivate you, or rolled back into the decluttering effort to fund decent replacements for things you are discarding.
* Don't try to do everything at once. After a while, decision fatigue sets in, then you tend to make poor choices and exhaust yourself. Cut big projects down to manageable size. One box or one cupboard is a feasible goal for most people. If you have more energy you might try a whole category, like kitchen utensils, where it's easier to reduce and reorganize when you can see everything at once. If you have less energy, do 1-3 items.
* If you can securely save money without it being raided for emergency funds, then you can sell off some things and reserve that money for later buying something else in the same category that you'll like better. This technique works great for any mass quantity of similar things in varying quality. That was how I eventually pried loose my tendency to keep every book that entered the house whether it was good or not, back when review copies were arriving by the boxload.
* Pile up the things in one category, look at them all, and compare that to the estimated maximum load. Often the pile is bigger than what you really need. Discard the least appealing items first. Frex, not long ago we cleaned out some of the cupboards with glasses and plastic cups, and found a lot of stuff we could get rid of.
* Condense, discard, and replace. Sometimes things accrue over time because none of them are quite right for the job but all of them are useful -- except that a proper set would be both more effective and more concise. Examples include food storage tubs and miscellaneous tools like screwdrivers. Identify what you really need, sell or give away the old stuff, and buy one new set of sufficient diversity and quality that it will do the job for a long time.
* Items of sentimental value are harder to part with. So, there's a refugee trick with energy where you aim to move the sentiment from one item to another, distilling it down to smaller and smaller amounts of stuff. (This makes it harder to part with the last few items, but most folks can manage to hang onto a few things.) Again, start by pouring off the energy from less-favorite items. Aim to keep things which have the most sentiment in a small, preferably durable package such as jewelry or stone paperweights.
* Look over the possessions in stages as time passes. With things from a deceased relative or broken relationship, often it's impossible to part with anything at first, no matter how ordinary. Later, you might start letting go of towels or plain dishes. Later still, you're likely to release things that are more memorable but not precious, such as spare chairs or tables, a painting that was always there but not a favorite, etc. There will probably be a few things you hold onto permanently, and that's usually okay. Prioritize mindfully.
* Allow time to feel your feelings. If you're just decluttering, you may find yourself mulling over the past in general; use this opportunity to revisit the memories before moving on. If you're clearing out the remnants of a familial or relationship loss, it's likely to bring up a lot more. Keep kleenex handy, and it's better if you have a friend present for practical and emotional support. You know how people sometimes offer, "If there's anything I can do to help, let me know" ...? Well, this is a thing that they can do to help, so ask. Sometimes ritualizing the process a bit can make it easier to bear, like putting precious things into a Keep box or going out for ice cream afterwards, allowing you to pack the feelings away after you're done dealing with them.
* Think diversity. It is usually more important to have a bunch of different things than a whole lot of one category. You need at least one Phillips and one flat screwdriver. More will be useful if they are different sizes. You don't need more than one of the same size, unless you frequently have people doing the exact same job at the same time thus requiring multiple matched tools.
* Think usage and priorities. Only mass-stock things you need a LOT of, or things you feel a strong need to stockpile. We have stacks of freezer cartons because we use them all the time. We have piles of spare bedding because I collect it, but also because sometimes the house is freezing and sometimes we have several guests at the same time. For most things, one or a few will suffice.
* Think crafty. You need fewer things, if the stuff you have on hand can be used to make a wide range of specific stuff you might need later. Tools and supplies are useful to keep around. Remember that it's legitimate to keep things if a practical use may be found for them, which also saves you money because then you don't need to buy as much stuff. Upcycling is your friend.
* If you're any good at organizing, do that too. If you can't find it when you need it, then you might as well not have it. I'm a good packrat; I can find anything, plus or minus 10%.
* Always take steps to motivate and reward yourself. Scale the size of the motivation or reward to the size of the project. Decluttering is HARD WORK. Give yourself credit for doing it, even if you can only do a little at a time. Every little bit helps.