Making a promise to fast if one's wish is fulfilled
Q: I had promised Allah Ta'ala that if He blesses my sister with a child when she was unable to conceive, I will keep 7 fasts for Him.
I even promised Allah Ta'ala to fast for two more reasons which He accepted and fullfilled.
I owe a total of 9 fasts for many years. I am trying but I am unable to do fast. Is there anything which I can do easily instead of keeping 9 fasts? I don't want to upset my Allah Ta'ala as He has done so much for me.