What is the ruling on abandoning medical college after performing الاستخارة (Istikhara) prayer concerning it, and does the questioner bear any sin for that?

If you sought Allah's guidance through Istikhara and then were unable to complete your studies at the first college and transferred to a second, you bear no sin. Rather, perhaps this is what Allah chose for you, and it is good, for Allah decrees nothing but good for a Muslim. Persist in performing Istikhara in all your affairs.