Questions about dental issues. How long fillings last etc....?

I have 20 fillings in my teeth. All back teeth. My teeth are very beautiful and I want to keep them, but I don't understand why I keep getting cavities? This has all "supposedly" happened in the last 5 years. I am 28. I never had a cavity until 5 years ago, and every six months it was 3 more to fill. My dentist has had me on that fluoride prescription toothpaste for a few months. She said one of the fillings was pretty deep, so when my filling falls out I may need a crown. How long do fillings last? Do they automatically leak and get decay under them? Will this script toothpaste help anything? I eat very well, get plenty of calcium, rarely indulge in sugar. I just don't get it. I floss daily and brush 2 minutes 3 times a day. What is going on? Anyways, How long can I expect these 20 fillings to last, and *IF* down the road I get a crown or something, how long do those last? Will I be able to keep these teeth into old age, lol... Or am I destined for dentures?