It was announced on Monday night that Harte, who was appointed Louth manager in November 2020, is leaving along with coach Gavin Devlin. Harte is set to take over as manager of the Derry footballers.
Harte led Louth from Division 4 of the National League to Division 2 along with their first appearance in a Leinster final since 2010.
"For the three years, Mickey Harte has given Louth football unbelievable service," Fitzpatrick told LMFM.
"He got us from Division 4 to Division 3, a Leinster final. He's done a lot but I will be very, very honest with you... I've got a good personal relationship with Mickey and Gavin.
"It was a real, real kick in the teeth. I didn't see it coming. We've been preparing for the last number of months with Mickey and Gavin, they've given an extra programme to the players. The players were fully behind them.
"We wanted to excel next year in Division 2. Even two or three weeks ago, we were talking about how do we get into Divison 1 and what extra resources we need.
"We had a thing in Louth, 'What Mickey wants, Mickey gets'. For the last three years, no matter what Mickey Harte and the management team wanted [they got it].
"Honestly, feel I let down. I'm just disappointed for the players."