When I was 15, on my first Roza, I drank a tiny sip of water. I can't hold this burden on my heart and was wondering if I can be forgiven?
as salam alaikum
you should ask forgiveness to Allah and sincerely turn to Him: He is the All-Forgiving. The Qur'an says: "Do not despair of the mercy of Allah. Surely He forgives all sins" (39:53).
If you are remorseful for your mistake and you do not intend to repeat it again, put your full trust in Him and continue to look for His mercy that embraces all things, insh'Allah you will be successful.
With prayers for your success.