I'm really bad at constantly restarting the game when it goes horribly wrong (I know, bad Daem). I love the M&B series but have never managed to "complete" one by ruling all of Calradia.
I've found recently that no matter what Kingdom I play, they always get absolutely demolished. I'll include a list below, but have the Devs added something where it nerfs the Kingdom a player joins to stop a power play - or am I doing something really wrong?
In order to know what you're doing wrong, I'll have to know what you're doing.
In short--probably. I united all Calradia during my first playthrough in December so it is very much possible but it requires a lot of prep work that you may or may not have done and then, once you've secured a country, gradual expansion until you reach a point of snowballing where you'll rapidly unite the continent.
So, in other to help you, I need to know what your playthroughs looked like because you either A; tried to establish a country too soon, too weak and poor to sustain it; B; established a country, but then tried to expand it too quickly; C; expanded, but in a bad spot that made you vulnerable; D; failed on a strategic level, causing a gradual recession that culminated in downfall; or E: inherited control of a broken country and lacked the ability to effect positive changes, and then proceeded to do the above mistakes without first restructuring and fixing the country you took over.
As for anti-unification roadblocks; in-game, there's a hidden threshold where, if you or an A.I. faction crosses it, every other faction will want to wage endless war with the hegemonist until its reduced to a relatively small size where it's no longer registered as a threat.
As for anti-player annoyances; when you take over a city/castle, NPC lords will typically donate "extra" troops from their retinues, but they're stingy when the fief counts as the player's, which makes conquest slower because NPC armies will likely leave an empty or mostly empty garrison unless you luck out and manage to assign the fief to an NPC clan before the army departs (in which case, it'll donate plenty of troops to tide it over). NPC rulers (including ones you serve) don't have this problem, even if you ultimately end up owning the fief, since it counts as the faction ruler's fief until its ownership is settled and when the faction ruler isn't you, NPCs are more generous in donating troops. There are several ways to counteract this annoyance; #1, bring your own extra troops to stuff the city/castle with (slow but steady); #2, wait until voting's done while in command of an army before entering/exiting the fief repeatedly (not as slow and steady); #3, conquer quicker than they can reconquer, eventually beating them through speed (fast and risky, but prone to turning it into a game of wack-a-mole unless you secure a corner, then a half, and basically make sure enemies/danger is in as concentrated a position as possible).
As for dealing with terrorism--as long as you never make peace with them, they'll never have enough Influence or money to retake lost territories but they will try to go for newly captured/highly vulnerable lands. Therefore, simply ignore them and never make peace and they're merely an annoyance that devastates the property values of their former faction capital. If you'd like to postpone dealing with terrorists as long as possible, a viable strategy is to leave factions with just one, isolated castle before ignoring them since they're totally helpless in that situation and, if you give broad policy orders to be Aggressive towards one other specific kingdom but be neutral/defensive with regards to this micro-state, they'll most likely ignore the micro-state and focus on the one you ordered an aggressive focus on. Note: it's better to order Aggressive on landless factions so that patrolling retinues will aggressively pursue them since the option also affects the rate at which your units pursue as opposed to ignore.
I think I've covered the most common problems/annoyances, if I've helped then let me know, and if you have something more specific that you can share it'd help me help you better.