The 29-year-old has combined both jobs for the past two years and last season was the only part-time player in the Football League.
With one year left on his contract Rainford asked manager John Still to release him early, a decision he said was incredibly difficult to make.
"In my 10 years of playing senior football it's easily the hardest decision.
"Sometimes the hardest decisions are the right ones, I'm a local boy so it was a big decision for all my family but deep down, as hard as it is, I feel it is right."
I do want to return to non-league football but at the same time I've played the best football of my career
Dave Rainford
Rainford's time at the south-Essex club coincided with their promotion from the Conference and their first ever season in League football.
And he said he enjoyed every moment of it: "It's been better than fantastic for so many reasons.
"The football, the fans and how people have taken to my family and the team-mates who are now proper friends not just team-mates.
"It's been such a good time and as hard as it is I just feel it is the right time."
The former Colchester United trainee added he was not about to turn his back on the game.
He confirmed: "I do want to return to non-league football but at the same time I've played the best football of my career, so I'd like to go to a club that are ambitious.
"I'll have few weeks now with the family and then hopefully pick a club and you never know it could still be in Essex."
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