Costa has failed to show the form of last season when his 20 league goals propelled the Blues to the title, netting just three times this term.
He is confident the threat of losing his place in the Spain squad for Euro 2016 in France will push him to get better.
"I have to improve a bit. The start of the season has not been good for me, not in how I have played nor in goals," he said.
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"The truth is Chelsea haven't been in a great moment, above all losing the other day at home (against Bournemouth).
"It was important to win (against Porto) to make the fans happy and to recover our confidence," added Costa in reference to Wednesday's 2-0 victory that took Chelsea into the last 16 of the Champions League.
"I know I have to be better to go to Euro 2016 with Spain. I have to work hard and fight but I know I have time to improve and I hope I will."
Costa also refused to rule out a move back to former club Atletico Madrid.
"You never know, look how well Filipe Luis is doing (with Atletico) now," Costa said of the former Chelsea defender.
"Life is full of surprises. I always wish Atletico the best and I always follow them and support them from afar."
Jose Mourinho's men are down in 14th place in the Premier League and travel to surprise leaders Leicester City on Monday.
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by Tony Jimenez)
