The France international defender agreed a four-year deal at Camp Nou after leaving Valencia, but injuries hampered the early stages of his time at Barca.
Mathieu has since settled into Luis Enrique's side and impressed during their 3-1 victory over Villarreal in the Copa del Rey semi-final first leg on Wednesday.
"From the beginning I felt very comfortable in the team," said the 31-year-old. "At some point I lost a little confidence, but in practice I try to give 100 per cent.
"I've had a bad time but now I'm back to my level. I'm happy with my game against Villarreal."
Meanwhile, Mathieu says Barca must not start dreaming of the final just yet with a tough second leg at El Madrigal on the horizon on March 4.
"Villarreal have a good team and are very strong collectively," he added.
"Our team has incredible patience, though, and we will move to contain them."
