Sirotkin made his F1 debut with Williams this year but will not continue with the British team, which has signed Robert Kubica to partner Formula 2 title favourite George Russell for 2019.
His backer SMP Racing says it decided to split with Williams because of its poor performance and development rate.
Sirotkin said he learned of that call last week but held out hope something would change, until Williams informed him on Wednesday night it would announce Kubica the next day.
“I was informed from the team yesterday night that its happening, but already last week I had a few talks, a few meetings with SMP Racing,” said Sirotkin.
“We analysed how things are moving, and I think it’s been quite obvious after those meetings that we don’t really want to take part in it anymore.
“So, already from the last week I kind of knew it’s going to happen, I had the week to prepare myself.
“I could not prepare myself. I still can’t believe it is happening. It is a tough moment for me.”
Sirotkin has scored a solitary point in his rookie season and Williams is set to end the year last of the 10 teams in the constructors’ championship.
The Russian said it “wasn’t worth it” to continue discussions but said this was not specifically because of a lack of faith in the team’s competitiveness next season.
“It is never one thing,” he said. “It is always many, many things on top of each other, which made us think that way.
“As a racing driver, I didn’t care about it all. All what I cared about is to be on the grid and, whatever the car is, just do the best job I can.
“I still had a strong trust inside that probably things will turn around, probably something will change, and it [losing the drive] won’t happen.
“But nothing changed and it happened.”
Sirotkin cut a dejected figure in the paddock on Thursday and admitted he felt “very, very sad” to be exiting F1 after one year with a poor car.
“I honestly think that we did a good job,” he said.
“The way we started, how we reacted to that, and how we’ve been as one united team, I am happy.
“I am also happy how I am performing under such conditions.”
Sergey Sirotkin, Williams Racing, on the grid
Photo by: Glenn Dunbar / LAT Images