Marshall was formally granted a release by the Auckland Blues on Wednesday and hopes to return to the NRL next month, with Cronulla his most likely destination.
The Kiwi league international flopped in brief appearances off the bench for the Blues in the Super Rugby competition and was on the verge of being dropped to club rugby in Auckland.
That prompted a meeting with Blues coach Sir John Kirwan in which both men agreed Marshall's switch to rugby wasn't working and never would.
"Why not tough it out? I don't think it's (about) toughing it out," Marshall said on Wednesday when asked why he didn't stay and fight.
"I think we're pretty courageous to stand up and say 'this hasn't worked' because as a stubborn person and someone who wants to succeed in everything they do, this is the first time I've had to say I haven't been good enough.
"I've had doubts the whole season about whether I was good enough or not.
"This is the first time we've (Marshall and Kirwan) been straight up honest and said that it hasn't worked out as we wanted.
"We both weren't getting what we wanted out of the deal."
Asked where he thought he had failed as a rugby player, Marshall replied: "When you play the positions I played – 10 and 15 – there's a lot of pressure.
"When I was under pressure I resorted to what I knew best and what I had instilled in me and that's playing rugby league.
"I felt I was playing a different game to everyone else. I was playing like a rugby league player with 14 rugby players.
"That made me realise I haven't picked up the technicalities of the game quickly enough.
"I thought it would be a lot easier to pick it up than it was."
Despite rumours linking him with a return to the NRL with Cronulla, Marshall insists no deal has been done.
He said: "It's pretty funny how I can join the Sharks without talking to them. I said to my manager I don't want to talk to anyone or know anything until this (the release) was out of the way.
"I've got nowhere to go at the moment (but) I found hunger (in Auckland), got fit and got motivated and feel I am leaving a better person."
Reminded of his comments last year in which he said he'd never play against the Tigers, the 29-year-old smiled: "It is hard to play for a club that doesn't want you.
"When I made those comments I was emotional. I've still got a massive soft spot for the Tigers but it would be rude of me to think I could go back in there with how good they're going."