Robertson was far from his best as Scotland were well beaten 3-1 by Ukraine at Hampden on Wednesday in their first match since Russia’s invasion of their homeland.
Ukraine will now face Wales on Sunday for a place in Qatar later this year, while Scotland’s wait for a first World Cup since 1998 goes on.
Robertson attracted criticism for attending Liverpool’s bus parade to celebrate winning the League Cup and FA Cup on Sunday, just hours after losing the Champions League final to Real Madrid.
The Reds had been in contention for an unprecedented quadruple going into the final weeks of the season, but also missed out on the Premier League title to Manchester City by a point after a thrilling final day.
“Personally, it’s been the toughest 10 days of my football career, of course it has been,” said Robertson.
“Emotionally, mentally, physically, everything, it’s not been great.
“But to be honest I will deal with that myself, I will go away and think about that myself.
“I am just gutted for those lads in there. Obviously I was desperate for them to get to a World Cup and play on the biggest stage and unfortunately we have fallen short.”
Robertson’s marathon season is not over yet as Scotland must now pick themselves up for three Nations League games against Armenia home and away either side of a trip to Ireland.
Steve Clarke’s men ended a 23-year wait to reach a major tournament by qualifying for Euro 2020 through the Nations League.
And Robertson said they have to learn from a limp performance against Ukraine to not let another chance at a major tournament slip by.
“We need to be ready the next time it comes around,” he added.
“The way it is with international football, people get older and you never know when your last chance of it will be. That’s why it hurts so much.”