The former QPR man became United's record signing when he arrived in South Yorkshire last summer, but made only 11 appearances - including three starts - in the Premier League for the Blades and joined Forest on loan late last week.
Forest have the option to make his move to the City Ground a permanent one, and Freeman admitted: "It was a tough year not getting the chances to play as regularly as every footballer wants to [at United].
"But I am very grateful for those opportunities to perform in the Premier League. I have made some great friends in the players and staff there and wish them and the fans the best of luck for the new season."
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Freeman struggled to displace John Fleck on the left of United's midfield and a late-season injury - together with the impressive performances of Ben Osborn in Fleck's position, while the Scot was sidelined - may have pushed him further down the pecking order at Bramall Lane.
“It was a similar situation at Arsenal," the former Gunners trainee told The Athletic.
"I left there because I wanted to play regularly. I wanted to have that chance. It is not quite the same this time but there is a similar desire.
That has always been the priority — to do everything I can to make sure I am playing.
“I would not say that I have a point to prove, but there were times when I was not playing and would obviously have preferred to have been involved."
Freeman says he consulted Osborn and Jack Robinson, who joined United from Forest, before making the switch.
“I was desperate to get out and play," he added. "It did not feel as though there was a better club for that than Forest.
“I have been very lucky during my career to play regularly almost all of the time. When I had played, I had perhaps started to take it for granted but I will not do that anymore.”