Using CIO method for night and nap time for 5 month old co sleeper, why is nap time harder for me?

My son has been co sleeping with me up until last night, i want him to self soothe and he did well last night only cried until the first 15 minute interval and then fell asleep for 4 hours and then went back to sleep with pacifier and slept about 9 hours. But his nap time he has been crying for 3 hours and only slept ten minutes ..what should i do..why is this harder and im being to hard on him? i know you should be consistent is it just because this is his first time trying it?