Both devices are great I was the same with the S4. I missed alot of features of the One (speakers, power button [for me I find it easier than the S4], HTC Zoe, design aesthetic of dialler, contacts, calculator, music app [much better than S4's music app] and even the ringtones [i thought the selection on the S4 were not as good as the Ones] and few other bits).
But the S4 is a really fab phone in its own right. So just give It some time, and remember both devices do have compromises (yes I too would have loved a baby of the two - the 13mp camera in the ones shell) but that's not an option... so like the song says
'Love the one your with, not the ONE you just returned'..... 😛