Arsene Wenger's side need only one more win to secure a top-four finish following Monday evening's 3-0 win over Newcastle, although many fans are wondering what might have been having seen the Gunners move seven points clear of current leaders Liverpool at the top of the table before a run of poor results against their title rivals.
But Mertesacker says a strong league finish and an FA Cup win at Wembley, which would end a nine-year wait for a trophy, would represent a positive season.
"We have struggled heavily with a couple of injuries, but we have regrouped very well and how we managed the situation was maybe more important than last year, so securing Champions League football was another test for us," said the defender.
"It is not done yet, Everton play Manchester City on Saturday, but we have it in our own hands.
"However, everyone wanted us to get further up, especially the fans, and we were top of the table for a long time. Everyone dreamed about the title, but it takes more than staying top until the winter period.
"We could not afford those (losses) in big games against big teams, so we did not deserve to be in the first three. That is something we have to manage much better next year.
"The (defeats in the) early away games (at Manchester City, Liverpool and Chelsea) killed us this year. Hopefully we can learn from that.
"Everyone questioned our qualities, but how we came back as a team, that makes us stronger for the future.
"We are in a cup final and it is important to secure Champions League football, so these are good targets to go for.
"We are happy to have a lot of players back. The squad is getting stronger and stronger."