If you're a utilitarianist, you don't sacrifice your life merely to save the lives of two or more other people with your organs (people who will, in most cases, be ordinary people rather than utilitarianists), except in the extremely unlikely situation that the people who need your organs for their survival are utilitarianists and there are no non-utilitarianist or less healthy and/or older utilitarianists than you who could save those utilitarianists' lives in your place!
The utilitarian thing to do is not "to improve the world more than an ordinary person would" or "to do the best you can at an arbitrary isolated point in time", but to improve the world as much as possible, in total, in the long run.
You can do so much utilitarian good during a long life as a utilitarian, that it outweighs by orders of magnitude the good your organs could do if you killed yourself while still in good health (with the exception of the above, extremely rare situation).
A lot of people falsely assume that in order to be a utilitarianist, you have to be very willing to sacrifice your whole life for just about any two other people in just about any situations, and that you have to give away almost all of your money and belongings to the poor and starving. As if every other human being would have the same value as you, despite your being a utilitarianist!
That's not utilitarian - not as long as very few people are utilitarianists. That's underestimating the rarity and exceptional instrumental potential utilitarian value of being a utilitarianist (alive).
It's almost as if Einstein would have spent all of his life taking care of mentally retarded kids, never doing any science - despite realizing his genius!
Or as if Mozart would have spent all of his life singing in bad choirs, never composing anything at all in his whole life.
It's utterly stupid! Those utilitarians. who think suicide merely for the sake of saving a few non-utilitarian lives is utilitarian, are so stupid, they should be ki- (just kidding).
The only things you have to "sacrifice" as a utilitarianist are things that aren't good for you anyway, like time-wasting activities and unhealthy habits, like watching sitcoms, eating junk-food, smoking, going on a lot of roadtrips all the time just for fun, pointless things like that.
To be a utilitarianist, you should probably eat well, exercise, maybe practice mindfulness, spend a lot of time learning stuff and being creative in your striving for having as utilitarian an effect as you can upon the world in the long run. Sound and rewarding activities like that. Certainly not suicide. You shouldn't even necessarily lead a very modest lifestyle, since that could prevent you from making a lot of money, for example. Making sure you always have a lot of money might be very utilitarian. Just think of how Elon Musk probably has a very utilitarian effect on the world by running his world-improving business, and how that would have been impossible had he given away all of his money all the time - or had he taken his life as a teenager merely to save a few other human beings with his organs, falsely believing that to be the "utilitarian" thing to do! What he now did was so much more utilitarian! (And maybe there was something even more utilitarian that he could have done!)
Also, I think Elon Musk seems pretty happy. You can have a very happy and long life as a utilitarian, since efficient world improvement can be fun and requires a lot of time (that is, a long life) - and you should have fun being a utilitarian, or else you won't last long as a utilitarian. Not lasting long isn't utilitarian!