Smalling admits Man Utd haunted by last season's collapse
The players thought they were comfortable, he admits, only to be stunned by Manchester City's turnaround.
But he is adamant United only have themselves to blame for surrendering their crown.
"It was a long season and there were spells where there was one team in front of the other," he said. "But we felt we definitely lost it because we got towards the end and we were far out in front and comfortable.
"It was very frustrating - to be so close and then miss out, especially in the last few minutes, it was one that really sticks with us.
"We will make sure we've learned the lessons from that and I am pretty sure that will never happen again.
"I think we've shown that when we get out in front, like we are now, we are going to make sure we stay there."