Let's cut right to the chase, to keep you guys from needlessly toiling through fields of text:

I always get stuck at the world-building part, be it urban fantasy, swords & horses fantasy or sci-fi. It's the thing about writing that I do best, the individual story lines, not so much. My question is, how can I resolve this issue? And now, for the fields of text.

Once, I had written about 100 pages, but 75% of that was background and appendix type of material, just 25% actual characters and story development, something that normally is the meat of a book. Then, I normally get stuck on that part and get frustrated, ending with me not writing for a while, losing the feel for the story and tossing it all away.

In my opinion there are two possible solutions to this problem. The first being that I negate the problem completely and find a co-writer who is tasked with the majority of the non-world-building, while I do the world-building. The upsides of this are that it is most likely easier for me, and that you write more when there are two of you, as general rule of thumb. The downsides are that you have to find someone and, more importantly, that writing is a hobby, not my job. I study Medicine, and that will take priority. As a result, I might not have enough time to put into it to satisfy my co-writer, and thus drag their productivity down. That wouldn't sit well with me.
The second solution is that I work on my weakness. This would probably be harder to do, but would make me beholden only to myself. It would probably involve short stories.

I have always liked to imagine worlds, be they Sci-Fi or fantasy, filled with stories. Lately, I have been trying to get those words on paper. As it turns out, I'm not good at writing big story-lines. I like to write about the back stories of things, the extra info on tools used, the discovery of technological, medical and/or magical advances, the creation of the world where the stories take place, and this is exactly what I did whenever I got stuck on the main story, to the point where the background was bigger than the main story. I simply like crafting worlds. Sadly, all this background info isn't really good material for being published in book form.

This gave me the following idea; 'What if I published all this on a website, with a tab for general world info, a tab about items, a tab about the rules, a tab with the personal stories? Would that work?' Upon browsing this forum, I read that anything published online, cannot be published in books as well. This makes sense, who would pay for something free, but since a big part of the stories about the world would be online, wouldn't that mean that everything else would also be unpublishable? Aside from all that, I'm also about to head off to the university, doing Medical (if I get accepted, but I have good hopes for it), so money won't exactly available in great amounts. I know that webhosting can be very cheap, a handful of Euros a month (I am Dutch), but I am still hesitant. Can you guys/girls help me out a bit?