In the last issue of the F1 Racing magazine, Alonso said he thought he would end his grand prix career in order to win the Indianapolis 500 and Le Mans 24 hour race over two Monaco GP wins.
Alonso competed in the Indy 500 last year and will join the 2018 F1 program with the World Endurance Championship, which will be joining Toyota in the first Le Mans race.
Alonso F1 Racing: "Yes, I thought I would change the series and leave Formula One." said.
"When I came back after last year's Indy 500 race, there were Austrian and Silverstone races and I thought, 'Maybe I can try different series for you, it might be best to try Indy and Le Mans to focus on three cadets (Monaco, Le Mans and Indy) .
"But I decided that it was not time yet, and it was not the right time after these results and these feelings."
"I could not live with this regret and bitterness in the rest of my life."
"I still want to be successful and we have an incomplete mission with McLaren, and I am confident that our course will change this year."
Alonso did not join the Monaco Grand Prix for the past year, and McLaren had allowed his Spanish pilot Honda to lose his motivation during his third season.
McLaren is on the Renault motor this year, and Alonso aims not to leave in a "hopeless" way.
Alonso said he would never be the ninth man in 2015 and 2017 to be the worst result for McLaren, the team's fourth or fifth place in the championship.
"We returned to normal this year, and it's great to know that we will be able to catch the podium even if we are getting ready for the races, pushing the first five and even doing a very good job."
"And if you can have a very special day, you can win even the race, I've missed this motivation and preparedness, but for the moment we are the first 5 and the podium is our main goal.