The Manchester City midfielder was sent home in disgrace after breaching quarantine rules when the Three Lions faced the same opponents in September, but credited Southgate as he redeemed himself with a brace in England's 4-0 Nations League win.
Foden told Sky Sports: "I was just determined to come back and do the best I can.
"[Being sent home] was one of the hardest moments of my life. That's the time you need the trust of your manager and Gareth had a lot of respect for me and it means everything to me.
"I just wanted to repay him with goals and do well."
Foden will "never forget" the experience of scoring his first international goals.
He said: "You are playing against the best players in the world each game, so I'm just really happy to get the goals.
"I won't ever forget it."
Southgate also praised Foden for the impact he's made after returning to the international set-up.
He said: "It's not easy to walk through the door after the way he left previously.
"But for us it was done. He's such an exciting player. It was a big week for him to come back.
"We all make mistakes. It was important for him to feel a clean slate.
"We know what he's capable of and to get the two goals was a glimpse for everyone for what he is capable of."