Finding out why they hate you
To find out why people don't like you, hover over the opinion number to find out why: (alas, the cursor isn't showing in this screenshot. I assure you, it's hovering over the number.)
You can find out why people don't like you from any screen that shows their opinion number. Here's another example:
Doing something about them hating you
Vassals often rebel because they don't like you. Once you've worked out why they hate you, try to do something about it. If you have 98% of the titles in a realm, then, as you mentioned in the comments, they probably think your demesne is too large. (Fun fact: "demesne" is actually pronounced like "domain." Language is weird.)
To fix both of your problems (demesne too large, nobles hate you when you give out a title), you need to give out a lot of titles at the same time. Your demesne limit is in the upper-right corner of the screen; for you, it probably says something like "20/5", which means your demesne limit is 5, and you have 20 titles. The "20" will be red, and there's probably a notification at the top of the screen talking about that (unless you made it go away).
You need to do three things:
- Pause the game
- Give away all your excess titles, until you're within your demesne limit.
- Give away some excess duchies, until you're down to 2. (You get -10 opinion against all of your vassals per duchy above 2.)
If you can, arrange it so that you have 2 duchies, and all of the counties within those 2 duchies, and you're within your demesne limit. Spread out the titles a bit, so you don't have one Super Duke who's going to challenge your power.
There's some other advice for preventing revolts over at this other Arquade answer, but this will get you started.
Many thanks to @svick for figuring out the actual problem here.
(As a side note, you probably also have some note about being the "wrong holder type" for your titles. You're a noble, you'll get the best benefit from holding castles (i.e., baron titles) and counties (count/earl titles). Give away your cities and churches to other people. (And when you hit your demesne limit, give away the baron titles and keep the count titles.))
Such a vast empire is bound to get split at some point, particularly after succession. You can either force it by causing.. an unfortunate incident, as Kyle suggested, or sowing dissent with a powerful king-level or powerful duke vassal of HRE who dislike their current liege.
Of note is that you can beat larger armies than yours with some clever warfare tactics. You are a pagan, and if the enemy hasn't researched Military Organization at least two or three times (unlikely, if you are that early in the game), then they suffer severe attrition in your territories. Also important is that hauling such large armies can take time, and would give you a few precious months of consolidating your troops and destroying some of their armies while they call their Occitan legions. Hopefully your realm is much more compact than theirs, and have ample naval access for quick transport on sea rather than by land.
If you deliver significant damage to them, factions within their empire might grow powerful enough to cause a civil war. This can give you a significant advantage and might cause them to ask for white peace to deal with troublesome vassals. This will give you some ten years of breathing room for you to expand and grow until they are able to strike again.
As you always have a casus belli against them, yielding some lands to them now without a fight might be a good longer-term strategy to reconquer them later when they are embroiled in another conflict -- particularly if you don't hold any of the counties in the duchy.
Best Answer
And the answer, in one word, is Proselytize. I had sent my chaplain there briefly, long enough to get the 'Majority population has converted to Catholicism' message once for each county, and then sent him back on his merry way to Rome. This didn't stick to provide lasting reform, however. You have to preach long and loud enough to:
That unlocks the option to upgrade tribal settlements to a castle, which you do by right-clicking the tribal settlement in the county holdings view. This decision costs no money or prestige. This creates a bishopric and city holding if there are enough empty slots (which is basically a free 1300-1400 gold), or converts an existing pagan temple to a bishopric if one exists.
However, you can only make this decision if you are the holder of the capital tribal settlement. If you leave it in the hands of the lacky or the pagan, they can make the decision in their own sweet time, potentially waiting long enough for the county to convert back to paganism, or be inherited by a pagan ruler. In my case, I had to murder an incompetent courtier I had installed who still hadn't managed to secure an heir and upgrade the holding myself, before installing a (hopefully) better lackey.