I too recommend reading Marie Kondo. She is a bit nuts, so take some parts with a grain of salt, and I also don't like tidying categories like she recommends, unless they are naturally in one single location.
However, like you, I dont like waste, and used to hold onto things because "they will be useful one day", "but this box is so pretty, it might come in handy", or "oh but ive paid so much money for this, i cant just give it away".
She has completely changed the way I think about these things now, but it's hard to explain here, you need to read the book!
You need to start valuing space that the clutter takes up - physical, as well as mental - more than the clutter itself. The money's already spent, they've served their purpose - it's time to let go.
I also really rate Dana K White and her no mess decluttering method. It's life changing, especially if you get easily overwhelmed/distracted or are time poor. It's slower than pulling everything out, but you don't have to deal with "It will get worse before it gets better" part. Look it up on YouTube, she's got loads of videos.
So start with easy things - bin stuff that's clearly rubbish, put things in charity box that you clearly don't need AND plan that charity shop trip. You need build some momentum so start doing SOMETHING, even if it's trivial. Good luck!