Masturbation While Fasting

As a teenager I had a bad habit of masturbation, which I sincerely and deeply regret. The urge was very strong and I resorted to this sin even in the month of Ramadaan while in the state of Roza. I would keep the fast as usual, i.e have sehri, and abstain from food and drink until the time of iftaar, but on one occasion would commit this heinous sin. May Allah Ta'aala forgive me. Ameen.
This evil continued for approximately 8 - 10 yrs since I became baligh. Thereafter Allah Ta'aala assisted me to refrain from this sin. Alhumdulillah.
It is a source of constant worry for me, and I know that taubah is not enough to eradicate this evil deed. I know there is qadha as well as kaffarah that needs to be kept. I am however perplexed with how to go about this. My concerns are as follows:

1) Do I need to make qadha for each fast missed as well as Kaffarah or is it just Kaffarah?
2) I am not sure of the number of days that I missed so how would I calculate the qadha?
3) What if I cannot manage to keep the qadha or Kaffarah fasts, I barely manage to get through the normal 30 days of Ramadaan and endure much hardship in this period?
4) Would it be permissible to pay Fidya and Kaffarah instead of keeping the fasts?
5) If so would I have to pay Kaffarah per fast or per Ramadaan?