Once you have signed a contract, you are bound by it. Lewandowski was obligated to play another year at Bayern, but he launched a media campaign to pressure the club to let him go before his contract expired. It was stupid blackmail and unprofessional behavior.
The result of the group stage looks all the more beautiful now: Bayern go to the playoffs, and Lewandowski, together with Barcelona, is sent to the Europa League.
Regardless of what was done, it has passed and Bayern have also had the advantage of the sale, so there is nothing for Lewandowski to regret.
He had already decided to move so there really was no other reason for him to feel sorry now because this was his choice.
All he has to do is focus on Barcelona although indeed the current slump to the Europa League is the biggest opportunity for them.
It is clear that it is impossible to change the past and we must look forward. But I think you will agree that Barcelona is close to being titleless this season and it will be a very unusual situation for Lewandowski as he is used to getting at least the German championship every season + scoring a lot of Champions League goals until the late stages tournament. Given Lewandowski's age, I don't think he won't regret that one of the last years of his great career went like this.