I've been decluttering for the last few years. I was a bit of a hoarder anyway, then DCs came along with all the crap they bring with them. I lost my DM a few years ago, and my F last year. Both deaths saw a huge influx of stuff into the house (I didn't have time to fully sort through at their houses, so had to bring back any 'keeps' or 'maybes' to sort at home). With work (I'm self-employed) , running around after the DCs, and MH issues as a result of all the changes it has been a huge and ongoing battle with the clutter, despite hitting the tipping point of 'I just need everything gone from my house' a year ago. It is still massively cluttered after many many boxes of things leaving the house.
What has made the biggest difference to how I feel about it, is concentrating on getting the rooms/areas I use the most done first. I had been doing the areas that were worst first (eg spare room, utility, garage, loft), when actually making sure our bedroom, the living room, and kitchen/dining room are clear is better. Obviously the other spaces still need doing, but it has less impact on me and now I feel more positive about dealing with them. This was my CBT counsellor's suggestion, it had never occurred to me before!
I broadly agree with not buying storage now, but it is worth it for things that aren't going anywhere. For example, I've bought a lidded box for all the confidential paperwork we burn in our fire over the winter, a little caddy to keep the DC's hair braiding stuff in and another for all the different types of tape in the office, some fabric bags for all the small bits of cycling crap DH keeps by the back door, and an organiser for the hall to stop the jenga-like pile of things we kept there. They shouldn't make as much difference as they do (esp the tape box!) but it all just looks neater.
The ruthlessness has been the hardest to learn, but it gets easier the more you do. Particularly with my parent's things I have found it easiest to do several passes. I felt too much guilt and attachment to get rid of loads in one go. I also struggle with the feeling of waste as a lot is perfectly useable, but not quite good enough for charity so into the bin it goes. I'm not sure how to get over this, so just try and block it out 