Hey, this is my first post here but I'd like to provide some input and my point of view in regards to your worries. I am not a dental professional but I've had a lot of work done in my mouth over the years.
The reasons fillings may not last:
1. The cavities on the teeth were too deep and cleaning/drilling the teeth may have led too going close to the nerve or other part of the tooth that may lead to irreversible damage. The fillings may or may not cause future problems, root canals, crowns and the lot. But they may last too depending on the care taken while they were done.
2. The dentists success strike rate in repairing teeth with composite or any fillings is not great, and she was talking from personal experience which doesn't speak well for their skill and ability. Composite fillings should last way longer than 5 years. I've got at least 14 of them so I know they last indefinite time.
3. Mouth hygiene in one's mouth may not be adequate and other destructive habits like smoking, sipping acidic drinks, snacking food all day long not letting one's healthy saliva to repair the damage from the subsequent bacterial heaven that those habits lead to, every day for years, causes unavoidable further decay. The brave ones go back to the dentist regularly and pay over and over, to get more and more work done with the only good part being the cosmetic side of things as teeth never seem to get better. But having said that, one's own teeth, regardless of repair done, are better than any denture, implants, or whatever substitute is now on the dental market.
When a filling is done with the right dentist, your fillings should last longer than you remember about them being done 10-15-20 yrs+. My first dentist's fillings( 31 years ago) didn't last 3 years at a time so in the end he almost ruined my teeth by constant repair but one day I coped on and I switched to a way better dentist despite the higher price mark. He wasn't nicest as a person, he was better as a dentist. And the need for constant fillings repair and further decay stopped abruptly for years to come.
So if I was in your shoes I would discuss with your dentist why she feels the composites she placed won't last longer than 5 years?
If you get an answer of the sort that this is because of the average life span of composites, then I'll be switching dentists for any future dental work and cross fingers for the current ones to last.
And then start preventative measures to preserve your teeth.
Brush twice a day with fluoride toothpaste, floss once a day in the evening, use cavity and decay preventative measures like swishing 1/2 spoon xylitol sugar after each meal which also helps reduce bacteria, plaque and rebuild teeth naturally if used 4-5 times a day in this manner. There are also commercial xylitol products like chewing gums and mints but make sure you get the ones that are 100% xylitol. Use mouthwashes but there is science behind it what to use, for example some of them may make you mouth acidic which is no good but in combination with others like fluoride mouthwash they can have a positive effect instead. So just some mouth wash is not good enough.
The only reason fillings fail for no technical reasons is because the bad bacteria and decay in one's mouth is not eliminated, just temporarily isolated from the mouth by a filling but in many cases that's not enough. Plus you have to think about your healthy teeth not getting further damage.
Sadly no dentist bothers explaining preventive measures and brushing and flossing alone can't kill the bacteria. Not brushing and flossing has not caused decay in the rare lucky people( 2% of the population) because they are not susceptible to tooth decay. My first husband rarely brushed and used mouthwash from time to time. He had 0 cavities and no decay, no fancy teeth by genetics but strong and healthy. My 2nd husband brushes once a day and never flosses and he has 0 cavities too and teeth like pearls, beautiful... He goes for the odd cleaning although I've never seen any build up on his teeth. He has a gorgeous smile.
I brushed and flossed all my life and the dental work in my mouth never ended.
I would encourage you to google Dr. Ellie Phillips and her view on dental preventative measures. In her opinion dental decay is a disease that can be eliminated and she has published a successful protocol on how this can be achieved. It includes the use of a specific toothpaste, mouth washes and xylitol, all commercially available to purchase but she states the importance of which ones and why. I think she has a book too but I haven't read it. It's all on her website and one can read the reviews and success stories too. In her opinion following her protocol will help you preserve all your teeth for life as this protocol not just helps prevent but rebuild damaged teeth(won't rebuild dead root canalled/crowned teeth or filled teeth but will prevent future decay on filled teeth so the fillings last a life time)
Good luck. It's never too late and with only 10 fillings you are just a baby as much as dental health goes.

It's been only my personal experience and I would encourage you to research yourself but oil pulling helps tremendously in eliminating plaque on teeth, bad breath, bleeding gums, and the lot. I use organic oils, like cold pressed coconut and sesame oil and I swish a spoon before brushing morning and evening alternatively. I've been doing it for 15-20 minutes but 10 or even 5 min would be ok. It lifted years of cigarette, coffee and similar stuff I abused my teeth with for 20 odd years, the bad breath, including in the morning, the need of chewing gums or eating mints throughout the day was gone, as your breath stays fresh, and as I mentioned earlier, the bleeding gums I used to have while brushing vanished. I haven't had the need of a new filling or dental work done in 9 years at least.. My last cleaning lasted 5-10 min and the dentist just shook my hand in the end because he found nothing to really clean or do. And that is a mouth full of old fillings, crowns and the lot, including cosmetic work on the front teeth.(not so cosmetic as I had root resorption on my front two teeth as rare as it is, and they got doomed with root canals and veneers at some stage)
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