Does what I did, by refraining from benefiting my friend with what benefited me, and sufficing with suggesting another solution to her, make me sinful or selfish, and can fasting treat this behavior?

There is no sin upon you concerning what you mentioned about your friend. It is better to advise her with what is beneficial and to love for her what you love for yourself, for the Prophet, peace and blessings be upon him, said: "None of you truly believes until he loves for his brother what he loves for himself."

The way to reach this level is by striving against the self and seeking Allah's help. Indeed, good manners are acquired through habituation, and one of the best aids in refining manners is fasting.