Goals from Jeremy Mathieu and Luis Suarez either side of Cristiano Ronaldo's equaliser sealed Barca's victory at Camp Nou.
The win saw Luis Enrique's men climb four points clear at the top of the table with 10 league games remaining.
Ramos, partly culpable for both of Barca's goals, lamented his side's inability to take their opportunities – but feels they can still catch their rivals late in the season.
"We showed a great level in the first half, had a bit of control and had to put our chances away, which we didn't," he told Canal+.
"We should've killed the game off in the first half. That’s what we were lacking in the game.
"We've let them get away from us. Four points is nothing towards the end of the season.
"There are still games to go and we're going to keep fighting towards the end of the season."
Mathieu headed Barca in front in the 19th minute, but Ronaldo levelled when he got on the end of Karim Benzema's first-time backheel.
Suarez's goal in the 56th minute would prove to be the winner as the Uruguayan took a long Dani Alves pass before finishing well.
Ramos said the key for Real would be to take the derby passion into every other game.
"We need to take the intensity from this game into future games. This type of game requires a level of concentration that's very high," he said.
