England entered the RBS 6 Nations finale unbeaten and targeting a first Slam since 2003. But the were well-beaten by a rejuvenated Ireland.
Johnson's men still had the consolation of the Championship title after Wales lost to France but the former Lions captain has revealed his players were left heartbroken by the loss.
"Let's take it on the chin, they outplayed us. We had a horrible first half," Johnson told BBC Sport.
"We came to win and we've fallen way short. This is a scar and we'll have to wear that scar.
England fielded a side containing 10 players aged 25 or under with only a handful that had won silverware at international level.
While Ireland boasted a vastly experienced side that clinched a Grand Slam of their own two-year's ago.
And Johnson reckons his team's relative inexperience played a decisive part in their poor display and led to the loss.
"We compounded errors, we knocked balls on - pressure's the name of the game," added Johnson.
"We had a lot of guys doing this for the first time - not a Grand Slam decider but playing a full championship - and there were a few errors.
"We've got what we've got - a team of good young players. Everyone's got a win and a loss in them, you get that if you don't play well and we got it today.
"It was horrible today, our best two attacking moves in the second half ended with us giving the ball straight to them.
"If everyone makes one mistake, that's 15 mistakes and it was that kind of day. It's just disappointing, we could have been better."
Nick Easter captained the side in the absence of Mike Tindall and he admitted the best team won on the day but he insists there are plenty of positives England can take from their Championship
"Ireland were the better team," said Easter. "We were second best from the get go.
"We won the kick-off and I think that's about the only time we were on top. We were playing catch-up in the second half but it was the first 20 minutes that really killed us.
"We had a lack of urgency and lack of discipline at times and unfortunately we didn't deliver on the day.
"They played very well and we didn't execute well, but it's a young team, we've got to learn from it and learn pretty quick.
"We missed out on the big one but this gives us some sort of reward for the strides we've made this season.
"Next time we'll be tested will be in the World Cup, so we'll find out a lot more then."