well since you are pretty nooby, its obvious you play against noobs too who have no idea about lane dynamics, so they will keep pushing into you and keep "poking" you under tower and dying to ganks.
truth is, if they dont push fast, wave will be in your tower only by the fourth wave, meaning at about 4:10 or so. even tho this gives junglers an opportunity to gank, playing a bit careful around usual jungle timings and placing your ward thoughtfully actually gives very good safety against this first, predictable gank. after the wave pushes back, he can simply zone you and trade with you whenever you come to farm. also, if he managed to push in correctly, the wave will now push to HIS tower and he can freeze easily, since nasus has little means to push the wave in fully except with his spiritfire which you, most likely, didnt max.
now the lane is frozen and he has a much much MUUUUCH stronger champion like renekton or darius or garen or wukong or olaf or basically anyone except soraka.
what will you do now? you are level 4 now. hes level 4 too. you have lvl 1 spirit fire. maybe lvl 2. you cant push the wave in fully, he freezes. if your jungler doesnt come to help you, it will stay like this for the rest of the game and you can never get a single cs anymore without instantly getting super outtraded or even dying.
if your jungler comes, sure he can push the wave in for you. if its a darius or renekton, he might even go and doublekill you depending on how long your jungler took to come and how stupidly you tried to trade. like if he is lvl 6+ allready and your jungler isnt lvl 6 (which is likely) or is a weak jungler, its still problematic and the enemy jungler might show up too and you have literally NO chance.
also, if you ever get the wave reset, hes allready even stronger than you and has more gold, so now he will just push you in and re-freeze the wave, and you will be in the same position.
your best shot probably is rushing frozen heart or iceborn gauntlet and maxing spiritfire so you can farm from afar and push the wave in fully sometimes to break his freeze. even tho this will deny you alot of Q stacks, you still get to stack it a bit and you will at least get some farm and are not removed from the game entirely.
tho some strong melee champions outscale nasus pretty hard, like darius or garen. so if you cant get infinite stacks, you will never be able to fight them in the game and their teamfight potential is much higher than yours.
this means, if dealt with properly, nasus has incredible problems due to his weak body and bad kit.
his power tho is against another weak fighting-champ (mostly ap champs) who have trouble either pushing the wave in fully so he can freeze, or trouble freezing so they cant really deny him. also, since most ap champs only deal their damage through "poke" and cant really just go and kill nasus (like darius and wukon can), he sustains everything up and outsustains their mana decently, allowing him to free farm and eventually win his lane.
having said this, the conclusion is that nasus is a "counterpick" at best but definitely nothing you should depend on.