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 makes decisions they think are 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 can win the Champions League. They couldn't win the League and the UCL, but the club won two Intercontinental trophies UEFA Super Cup, and the 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.
However, I think PSG's performance improved greatly after the departure of the French striker. It seems many players rediscovered their potential immediately after he left. Before now, the club was giving him all the attention, while other players were neglected. There are also rumours that Mbappe was selfish on the pitch and also caused division. But we have more selfless and united players in the French outfit now.
Let me start from the last comment made on Mbappe being selfish, i think he is not to an extent, because if you're to look more closely into his kind of playing strategy, he is used to using such pattern and moreover, he is still one of the most outstanding players of this time, so there must be a unique means of identification which may be attributed only to him, but i wouldn't want to support on what was behind his performance and intention to be being selfish for his own interest, working as a team could help them all achieve a common goal.