Coventry looked a class above Port Vale in the first-half and the home side were fortunate to be only one goal behind at half-time.
But it was a totally different story after the break and Vale substitute Chris Birchall's 82nd-minute clincher was ultimately thoroughly deserved.
Pressley said: "We didn't deserve anything for our second-half performance and it's a big disappointment because we were excellent in the first half.
"In the first half the boys carried out all the things we had been working on during the week. Vale liked to get the ball forward early and get on to second balls and we stopped them from doing that.
"But the distances between the lines of our team opened up in the second-half and that allowed Vale to put us under pressure.
"We didn't deal with that and have to show a more steely mentality, especially when we defend crosses into the box.
"I was really, really pleased with our first-half performance, but it was role reversal in the second half. We sat back to protect our lead and in the end that was our biggest problem.
"But we had gone six games unbeaten before we came here and we've made huge strides over the last two months.
"What we have to do now is keep working hard and keep improving."
Coventry dominated the first-half and the only surprise was it took 34 minutes before they went ahead through Callum Wilson's ninth goal of the season.
Vale's defence was caught square by Cyrus Christie's hopeful ball forward and Wilson sped clear to beat Chris Neal from 10 yards.
The home side drew level in the 40th minute when Chris Lines swung over a free-kick and Tom Pope rose superbly to glance a 10-yard header inside Joe Murphy's right-hand post.
But Coventry regained the lead in sensational style three minutes later. Franck Moussa surged forward from the centre circle and crashed an unstoppable right-footed shot into the top corner from 30 yards.
The substitutions sparked a massive improvement from Vale in the second-half as Williamson, twice, and Pope squandered great chances to level.
But it was now time for the Coventry rearguard to buckle and Vale skipper Doug Loft slammed in a low 10-yard shot for a 59th-minute equaliser after the visitors had failed to deal with another Lines free-kick.
There were further chances at both ends before Birchall's right foot squeezed a deserved winner low past Murphy after Williamson and then Pope had shots blocked.