The 21-year-old has not been at the peak of his powers his season due to a combination of a lack of form and injury problems.
Wilshere again found himself on the bench in Arsenal's last Premier League game at Manchester United and was a substitute in England's friendly defeat to Germany at Wembley on Tuesday.
And the midfielder is determined to rediscover his form in order to secure his ticket to Brazil.
When asked if he was thought he was guaranteed a place in the England squad, Wilshere said: "No, not at all.
"You have seen over the past couple of months the talent there is, even in the Under-21s.
"Ravel Morrison and all these players are coming through. You are never guaranteed, and cannot rest on your laurels.
"You are only as good as your last game, they say, so when the time comes, I have to be on top form, and there will be a lot of players looking to be as well."
Wilshere has scored twice in 19 appearances for club and country so far this season and will be in contention for Arsenal as host Southampton on Saturday.
