How long do fillings last?

I just had a crown put in 3 months ago I think b/c the cavity had grown (had a filling in there already) and they said my tooth would fall apart if they tried another filling; around the same time, I had a filling re-done b/c it was falling apart. I had another crown put in a few years ago. That particular tooth also needed a root canal b/c my dentist said there is "a slight chance that crown teeth roots will die."
Now, I just had a checkup and they have a watch on my last molar b/c that filling is falling apart and they said it may need a crown in my next checkup.
Aren't fillings supposed to last more than 5 yrs? These are white fillings. My mom has (I believe) silver fillings that have been there for 30 yrs! WTF? And, I have no dental insurance, plus the place I go to is one of the most expensive for fillings and crowns. My biggest question is, should I seek another dentist? I can't afford this s***! I take very good care of my teeth (they even said so). Are they just complete idiots? Getting this big procedures done is really stressful for me and I'm sick of this b.s. and am at my wits end, thinking of just going to someone else.
Help.