The TV star, one half of presenting duo Ant and Dec, said he wanted to speak out about his issues in order to help others.
McPartlin, 41, told The Sun on Sunday: "I feel like I have let a lot of people down and for that I am truly sorry.
"I want to thank my wife, family and closest friends for helping me through this really difficult time.
"I've spoken out because I think it's important that people ask for help if they're going through a rough time and get the proper treatment to help their recovery."
McPartlin is believed to have checked into a rehabilitation facility, where he will remain for up to two months.
The paper has reported that McPartlin's problems stemmed from failed knee surgery two years ago.
With an ongoing knee complication, McPartlin is said to have started taking prescription drugs to cope with the pain.
A source said that alcohol has also become a cause for concern among friends of the presenter.
His busy work schedule, which sees him, along with professional partner Declan Donnelly, front programmes such as I'm A Celebrity ... Get Me Out Of Here! and Britain's Got Talent for a large portion of the year, has been an additional factor to McPartlin's battle with anxiety.
Several stars have commended McPartlin for speaking out about his personal issues.
Apprentice boss Lord Sugar wrote on Twitter: "Very brave Ant to go public. This is the first stage of the road to recovery. All the best Ant it, will all be sorted for sure @antanddec."
Dame Kelly Holmes said: "As I say at every speech I do,we are all human! Best wishes to Ant @antanddec You will come back stronger,your fans love you #itsgoodtotalk."
McPartlin is the long-time professional companion of Donnelly. The two having forged a union after starring in children's drama series Byker Grove together in the 1990s.
They went on to enjoy a music career under the names of their characters PJ & Duncan, releasing songs such as Let's Get Ready To Rhumble and Shout.
After several years presenting a range of programmes, they landed a job hosting Saturday morning ITV children's programmes SMTV Live and CD:UK, where they remained for three years from 1998 until 2001.
Jobs on entertainment shows Pop Idol and Friends Like These led to their primetime career taking off in the early noughties, and they are now best-known for their stints on I'm A Celebrity, Britain's Got Talent and their own entertainment show, Ant and Dec's Saturday Night Takeaway.
Ant and Dec have won the TV presenter award at the National Television Awards for 16 consecutive years.
Last year, the Geordie twosome were awarded OBEs in the Queen's Birthday Honours for their services to broadcasting and entertainment.
McPartlin is married to his long-time partner Lisa Armstrong.