Your decision to donate is a noble one. And, I would like to encourage it. Yet, I also think you should talk more to your fiancee and mother, and not to go ahead immediately without their consent at this stage.
It is true that you make a decision on your own body; and not on anything which
is common (Prince Vessantara gave away treasures belonging to the kingdom, and
consequently was expelled from the kingdom by the people. See Vessantara Jataka.).
My advice is focussed on what would follow after the donation: say, harmony
within the family, and also about joy that should follow after the donation.
Of course, a very mature and developed practitioner like senior bodhisatta would
give up even their life to save others when necessary.
I hope this gives you some thought.