How can I deal with my family, especially my mother, who opposes my Islam, knowing that I do not want to disappoint them?

Congratulations on the blessing of Islam. Begin the path of guidance by memorizing the Quran and studying its rulings. Do not be surprised by your family's opposition; it is a trick from Satan and a test from Allah. Be compassionate towards your mother and supplicate for her guidance. Filial piety is obligatory, but there is no obedience to a creature in disobedience to the Creator. Do not pay attention to the words of others, for your record has become pure and clean. Invite your friend to Islam; if she refuses, then part ways or marry (it is permissible for a Muslim man to marry a Christian woman). Be kind to your family with good words and gifts.