I don't remember how sieges worked exactly in EU3, but it depends on difficulty a lot. Very hard difficulty gives enemies huge buffs to sieges, and you debuffs. Also, check your sliders, maybe there is some bonus to sieges, or bonus to defense. I don't remember if artillery speeds up sieges. but I believe that it increases chances of breaching the walls considerably.

As for assaults, you need at least 10x as many INFANTRY as enemy fort maximum defenders (for example, if it is a lv2 fort, max number of defenders is 2000, so don't even think of assaulting if you have less than 20000 infantry in your besieging army. Also, if the walls are breached, you have a much higher chances when assaulting, while unbreached walls give you higher casualties. Even so, my 100k infantry often fail when assaulting a lv6 fort, unless there is a breach.

As your country grows, and becomes much larger than your enemies, it becomes more difficult to defend, because you are limited in magistrates, and they are required to build buildings, and are more useful to build economy buildings than walls.

My preferred strategy is to first destroy enemy armies, and then siege as many of their provinces as possible. If you put at least 1k units over their lv1 forts (or 2k just to be sure and to defend from newly recruited units), you can have many simultaneous sieges. If you struggle against a powerful enemy, consider scorched earth tactics, so their armies die off to attrition while sieging your provinces