How I'm interpreting Kousei's advise: I think that when he says he wished he'd acted more selfishly, he means that he wishes that he had been more honest and upfront with his partner. It's a big part of Japanese culture to act selflessly and that selfishness is frowned upon for the most part. But Kousei, realised with his previous relationship that by chasing a dream headfast without being open and honest with your partner (aka. Selfish by a Japanese standard), you end up with regrets.
How I'm interpreting Kousei's advise: I think that when he says he wished he'd acted more selfishly, he means that he wishes that he had been more honest and upfront with his partner.
It's a big part of Japanese culture to act selflessly and that selfishness is frowned upon for the most part. But Kousei, realised with his previous relationship that by chasing a dream headfast without being open and honest with your partner (aka. Selfish by a Japanese standard), you end up with regrets.