However, he was proud of his players after a final day 4-2 loss to Newcastle United at home.
Frank chose to focus on the positives when speaking with BBC's Match of the Day, stating: "We know goals can change the momentum, but we made two big mistakes that basically gave them two goals.
"I've said before, these players are fantastic characters and they showed fantastic mentality to come back into the game. We were close to getting something, but we didn't.
"It's putting things into perspective this season. For a club of our stature, every season in the Premier League is a success. We had a good first season, an even better second season and an OK third season. We've learned a lot and hopefully we have the foundation to elevate us to more success."