Why did Johnston feel differently about the campaign against Sherman which looked like the equivalent of a Confederate disaster?

I can only assume that the asker is referring to the Atlanta Campaign. Johnston considered his campaign in Mississippi a failure and felt he was doing nothing in the Carolina's except annoying Sherman so the Atlanta Campaign is the more likely.

Joe Johnston felt that he had been quite successful against Sherman in the Atlanta Campaign for these main reasons:

1 - Sherman commanded a combined force of between 100,000 and 120,000 men in the form of his three Armies (the Army of the Ohio - John M. Schofield, the Army of the Tennessee - James B. MacPherson and the Army of the Cumberland - George Henry Thomas) while Johnston's highest number of soldiers, effectives of otherwise, was somewhere between 60,000 and 70,000 yet his Army remained a viable force between Sherman and Atlanta and still numbered around 60,000 when he was removed.

2 - Johnston had managed to do more damage to his enemy that he sustained. Though the casualties of 10,000 Confederates and about 15,000 to 20,000 Federals was small in terms of the losses suffered in Virginia they were hard fought losses and only the differing nature of Johnston and Sherman from Grant and Lee kept the casualties low.

3 - The Federals had, comparitively, penetrated shorter into Georgia's Territory than it had Virginia's and the cost of protecting Georgia for the Confederate had been less than that which Lee's Army suffered and the enemy had advanced at a slower rate in Georgia than it had in Virginia.