in his previous life he only had piano and grieve his relationship with his parents and now he fall in the same route with this new life. He even don't take the time to speak russian
I think he was sad that his relationship with his family suffered because he wanted to pursue music, but that doesn't mean he actually regrets pursuing music. He just wished he could still do music AND have his family. He loves the piano so much that he killed himself when he realized he could never play it again so that should tell you wat he values most in life. He's still trying to have a good relationship with his new family, which is why he begged his dad and didn't run away (so he already changed a part of his path in this life) but music is the path he still wants and always will want to take
they all have flaws and doesn't deny it. they want her but can't cross a line.
Yoshino is a boss, she can take them both and protect her heart too.