I think some sort of an inventory or checklist is a good idea for someone actively collecting - but which format to choose depends on your needs. I'm very much a notebook person, but I've always kept lists of my collection on my computer (Word or Excel files). I'm not very good at remembering backups, though, and have lost my files previously... It's not fun to start listing them again from scratch! Something like Google docs/sheets would probably be a better idea. But because I'm not a very active collector, and I rarely sell anything, a notebook might actually work very well for my needs. Thanks for the idea! 

Here are some features of each format that I can think of:

Electric (files offline or in cloud)
- Easy to edit, you will never run out of space
- Easily copy & paste to create wishlists
- Excel and similar spreadsheet options: you can have a 'book' with separate sheets for different collections (e.g. by generation, accessories, merch...) and can include information in columns --> easy to sort (let's say you include the date of acquisition in one column, so you can sort by this column and instantly know how many ponies you added to your collection this year, for example).
-  If you don't want to track any other information about your items except list what you have, a regular text file should work just fine (you can create headings for different sections for easy navigation).
- Backups needed if working offline --> need to develop a system to make sure info is up to date


Notebook (paper)
- Requires a system of organisation to easily find information --> I suggest adopting some of the bullet journal type methods, e.g. using page numbers and a table of contents / index
- Two different notebooks for ponies and merch?
- Perhaps not ideal for someone whose collection frequently changes (lots of purchases and sales) but would work for a less active collector or someone like me who mainly adds to their collection, slowly
- If you want to ensure you won't run out of space, maybe a checklist type of solution --> list all the ponies in the set and mark the ones you have (writing them all down is a lot of work, though!)
- Could also look into a folder or some other solution where you can add pages and move them around

I suppose the ultimate question to ask yourself is: What do I need to know about my collection? When and how do I need to access my lists?
For me, the main purpose of making lists is to a) avoid accidentally buying doubles, b) getting the satisfaction of seeing sets completed. The last time I started over, I also began to include information such as where I got the pony from and how much I paid for it - the latter so that it would be easier to see how prices fluctuate over the years. But now I've switched laptops again and I'm not sure if I have a copy of my spreadsheet anymore...

I love notebooks, but I know your dilemma of running out of space/not knowing how many pages to save before starting a new section etc.

How about a binder with index tabs? That way you'd have an index at the start, and you can move pages, add pages and sections, remove stuff you don't like etc. hat's what I would do, probably

eta: and if you're worried about writing an index in case you have to move things, I would suggest writing on washi tape (which is removable) or laminating the index page and writing with marker you can wipe off! 

I'm sorry you felt sick and I started having back problems when I was 17 too.

Kiera is so cute! The perfect thrift store kitty!

Ooh, answering the phone is a big deal! When I worked I never liked doing it.