Mbappe played for PSG for a long time but he could not win any major tournament, that is why he decided to come to Real Madrid last season and he thought that if he went to Real Madrid, he might taste the Champions League trophy in his first season but in reality that did not happen. PSG, which Mbappe left to join Real Madrid, qualified for the Champions League final this season and they have a good chance of winning the final match.
If PSG can win the Champions League for the first time this season, then Mbappe may regret it a little and may think that if he had stayed at PSG for another season, he might have won the Champions League trophy for the first time. However, it can be said for sure that he will have the most success in the club he is currently at.
Maybe the first season didn't go well, but the coming seasons will definitely go well for Real Madrid and Mbappe will have a good chance of winning the Champions League title for Real Madrid next season.
Everyone takes decisions they think is best for them. Real Madrid is one of the best clubs in the world and many players wouldn't turn down the offer of playing in that Spanish club. It is also perceived as a club that has the capacity to win the Champions League. They couldn't win the League and the UCL but the club won two Intercontinental trophies UEFA Super Cup and FIFA International Cup. Not winning the Champions League shouldn't make him regret his move to the Los Blancos because they will still be in the competition next season.