Setsuna learns how to play Haruki's guitar because she loves him and can't live without him even though he abandoned her.
To understand this answer better I highly recommend reading these two links about Setsuna and Kazusa.
How much does Setsuna Ogiso love Haruki in White Album 2 anime and visual novel?
Does Touma Kazusa love Haruki in White album 2 anime and visual novel?
The song she plays in her epilogue is quite relevant. It is "Snow Powder". This is a love song for Haruki.
It is a song quite different from her usual love song for him "Todokanai Koi" (which shows her unrequited love for Haruki).
In Snow Powder there's no longer any hope. It is a song where she says she will never forget the first time they kissed, she also says she still loves him, it is mentioned that he was kind to her (in the visual novel she always mentions how kind he is to her and that he's the only one to show such kindness)...
Also notice that the lyrics was changed to adapt to the new situation (the original snow powder song had different lyrics that could be addresses to pretty much anyone).
If you ask me the most relevant part is when she says she will never forget when they first kissed. It was the time she confessed to him and he accepted her.
Haruki had spent more than 2 years being Setsuna's boyfriend. He had told her she was the only girl for him. He had made her so many promises. He had asked her to marry him, and of course she had accepted (when she accepts she already knew he was lying to her and meeting with Kazusa behind her back. Even so she thought "As long as you choose me in the end everything's alright").
After spending 5 years in Europe Kazusa goes back to Japan. She had been thinking about Haruki all this time and suffering without him. She thinks that Setsuna had become her worst possible enemy as she had snatched the man that she loved. Then Haruki, who starts meeting her without telling Setsuna, must make a choice: Be truthful to the girl that had always been loyal to him, making her yet again the happiest girl in the world and the happiest ever since her birth (as she had said she was when he had accepted her feelings in concluding chapter) or destroy her and make her the most miserable girl in the world.
Haruki finally abandons Setsuna and both him and Kazusa decide to completely alienate from her. They go to Europe together and after some time they get married.
Setsuna had always wanted to be the number one girl for Haruki and to hog him completely. She had always been hurt because he paid too much attention to her and she knew Kazusa would steal him (because she had always been deeply in love with him).
This is one of the conversations that take place in concluding chapter. After 3 years fighting for him and crying countless nights because he wouln't accept her feelings he finally accepts. Setsuna thought her deep depression would finally stop. She would be the happiest girl because she had finally won. But this is what happens. She said she had been waiting since her birhday 3 years ago (when Kazusa was still friends with her) to give her body to him. He had told Setsuna he had forgotten about Kazusa. She is now the only girl for him and they can move on, he would stop ignoring her. But suddenly she sees a magazine on the bed. It was an article Haruki had recently written about Kazusa because of his work (he was a journalist and Kazusa a famous pianist). This is what happens:
Setsuna: "I've read this article dozens of times already"
"I've gone through it countless times. And every single time, I would start with a bitter smile, and then start laughing out loud as it got sillier"
"And by the time I finished, I would always be drenched in tears."
"Haruki-kun, have you really forgotten about Kazusa?"
Haruki: "I have... It took me a whole 3 years... And in that time, I made you go through some truly painful experiences, I'm really sorry..."
Setsuna: "What do you mean you forgot?"
"You're telling me you're different from how you were 3 years ago!?"
"I'm gonna snap now, ok?" (finally facing me with her red, swollen eyes)
"I've read this article dozens of times. And everytime I did I laughed and cried..."
"Both my heart and body ached unbearably... and I'd spend the night alone, sleeplesly."
"Because... what you're saying hasn't changed at all. It's the same as when you went chasing after Kazusa back then."
"It's so filled with love. Isn't it overflowing with your feelings for Kazusa!?"
"This... this article... it's the same as your words back then!"
Setsuna: "Having read such a touching love letter, how do you expect me to accept this...?"
"LIAR!LIAR!LIAR!LIAR!"
"When I came out of the bathroom just now... your face looked like that of a child who was caught red-handed doing something aweful"
"Even after all these years..."
As you know after that Setsuna finally calms down and a few days later she is sleeping with him and telling him she's never been so happy in her entire life because he finally accepted her feelings.
The next 2 years are super happy for Setsuna. After that he asks her for marriage but instead he chooses Kazusa.
Kazusa says:
I have become the happiest girl in the world;
It is the best thing that ever happened in my life. Truly
She also says she is the most fortunate girl in the world now and that returning to his warm embrace is the best thing that has happened to her.
At the same time she also says that even though it was at the expense of causing her friend to become the "most miserable girl in the world" (obviously Setsuna) she still feels tremendous happiness.
Personally I really enjoyed Kazusa's true ending. Kazusa showing Setsuna she wants Haruki more than anything else (by wanting to cripple her own hands, therefore ending her career) really surprised me. It also surprised me the game tells us that only with Haruki can Kazusa reach her full potential as a pianist (luckily her hands are fine), so she'll be a big shot. If something I didn't like that Haruki left his friends behind and Kazusa did the same with her mother (She decides to live in Japan because of her leukimia).
I didn't like Kazusa's normal ending though (in contrast with her true ending which I explained before). Haruki sleeping countless times with Kazusa yet being unable to break up with Setsuna was quite frustrating. As you know in the end Kazusa faces reality and gives him to Setsuna because he would be hurt in the future otherwise and she doesn't have the abilities to cure heart wounds (and Haruki's indeciveness sure make it seem he would be hurt if he abandons Setsuna. As a matter of fact he's already suffering from all the horrible things he's done).
Kazusa: Only she can heal you, I cannot.
Considering she leaves Japan and that her mother (who should die soon) lives in Japan and the fact that she says in Coda she can't fall in love with someone other than Haruki it's not the best ending for her.
Setsuna forgiving Haruki so easily for what he did to her is what she always does ( no matter what he does to her marrying Setsuna is always available to him). Not that it helps much though as if it wasn't for her this would also be an ending with Kazusa. She played victim hoping Haruki wouldn't break up with her and it paid off. Not that I'm against it, love is love and not being able to give up on him (like when she says in the VN: "I will never give up on him") is nice.
