Why does my home-built bookshelf keep falling apart, and how can I make it more stable?

I totally get why you're frustrated with your bookshelf falling apart - it's disheartening when a project doesn't turn out as planned. From what you've described, it sounds like the issue might be with the type of wood you're using and how you're assembling the shelves. Pine wood can be a bit soft and prone to warping, which could be contributing to the instability of your bookshelf.

I'd recommend trying a harder wood like oak or maple for your next project, as they're generally more durable and less likely to warp. In terms of assembly, make sure you're using the right type of screws and applying the right amount of pressure - you don't want to overtighten and split the wood, but you also don't want it to be too loose. I've found that using wood glue in addition to screws can really help add stability to a piece of furniture.

As a novice woodworker, it's totally normal to encounter some trial and error - I've been there too, and it's all part of the learning process. One thing that might be helpful is to try clamping your shelves together before screwing them in place, to make sure everything is nice and tight. You could also consider adding some extra support, like brackets or corbels, to help distribute the weight more evenly.

I hope these suggestions help, and I'm sure you'll get the hang of woodworking with a bit more practice - it's a hobby that takes time and patience to develop, but it's totally worth it in the end. If you have any more questions or need further guidance, feel free to ask, and good luck with your next project!