As a retired Nasus main, I kind of know how you feel. I played Nasus before the VGU or heck back when he has a voice actor that passed away long time ago. I had a massive field day of farming in the lane all day uncontested. But as time passes by as the game shifts into E-sports oriented which is kind of surprising all the several nerfs that he got due to being unfun to play against and getting buffed here and there, the changes overall has not been kind to him especially the whole runes/mastery fiasco. I enjoyed Lifesteal quints on Nasus so much that I actually managed to retire alongside of there are a lot of glaring weakness that he has.
So you picked Kai'sa into a Nasus. First off, that's a mucho bad idea. Nasus is an ADC's WORST NIGHTMARE for a reason, he WILL wither you and negate most of the damage you dealt to him. Did your attack speed got reduced as well? Well, back in my day it used to be twice as much potent. My point is that picking an ADC champion against a Nasus is perhaps the worst thing you've ever did and now that you acknowledge that mistake, I can tell you a couple of his weakness: His W and E cost mana. Almost 4 times as much as his Q. Not to mention he has to spend 100 mana to use his ultimate and maintain his 20 mana upkeep to maintain his mana. Pick a champion that are able to take advantage of his scarcity as a really good Nasus can determine lane control.
Secondly, it is true that he is vulurnerable to kiting. HOWEVER, it is IMPORTANT to know that he can only wither ONE TARGET. His Q, does shit ton of damage but let's be honest so can any other champion if used right. His E, being his only PROPER teamfight utility reducing armor anyone stupid enough to stand in. Caution must be taken against this when there is an enemy MF especially when fed. The main problem with Nasus is that in teamfights, no matter how much Nasus can be fed: There is NO WAY that he can reach your carries as the frontline him being the only threat. So the only thing you CAN do is to out utilize the Nasus by CC, kite, get your support to get
if Wither is that threatening, get your
to ult,
to hook and/or flay him away,
headbutts him away, you get the picture as your ADC which advising
(which should be in bot lane, not AGAINST Nasus).
If you want to excel against a Nasus, I suggest you pick someone who is and can be more potentially useful than him.
is VERY STRONG right now.
does true damage with a very high skill cap.
if you wanna fight fire with fire since he now stacks health.
is conventionally strong right now and can out utilize the Nasus later.
has a better teamfight engagement than Nasus.
can be hell of a kiter despite being withered.
is a hypercarry and Nasus is a joke compared to her. EVEN
is more useful than Nasus aside of being able to beat the shit out of him.
If you are quitting because of the ONE CHAMPION, I cannot fathom how you managed to play this game. Nasus has been around for a very VERY long time. The rune/mastery changes has quite left him a very low tiered spot. If you think Nasus is unfun to play against, you probably are not going to have a fun time with Irelia. Because there are champions that are a LOT worse than him (Relative to being not fun to play against). The other option is not go top lane at all if you hate him so much.
So TL;DR git gud and read, stop going top if Nasus give you that much of a hard time because there are champions way worse than him. If you are actually quitting because of an old champion that has been around for a long time and is shit right now, then I wish you luck.