If you are able to fast, then you must fast, and in this case it is not permissible for you to break your fast and feed others. Insulin injections do not break the fast; you can fast and take the insulin injections. And you have to make up the fasts that you have missed. See question no. 1319.
But if fasting will harm you, or it will cause you great difficulty, or you need to take medicine during the day, then it is permissible for you not to fast in that case. If you will never be able to make up the fasts in the future, then you have to feed one poor person for each day.
It is not permissible for you to pay the fidyah in cash, rather you have to give food, because Allaah says (interpretation of the meaning):
“And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day)”
So you have to look for poor people so that you can do what is required of you, or give money to someone who can buy food and make sure it reaches the poor on your behalf.
And Allaah knows best.