I've always noticed that immediately after consuming lemons, limes, oranges or ACV (apple cider vinegar), my teeth would feel "weaker" or thinner.  Below is a video by Foods4Health on healthy foods damaging your teeth, lemon and ACV are two of the foods listed.
Why do lemons and ACV cause tooth decay?
The danger apple cider vinegar poses to your teeth and gums comes from its high acidity. Braggs, the most popular brand of unfiltered and organic apple cider vinegar, clocks in with a pH level of 2.85. To put that in perspective, the pH scale runs from 1-14, any pH level lower than 7 is considered acidic. This puts ACV above soda, on par with lemon juice, and just under stomach acid and battery acid in terms of acidity.Anything highly acidic like soda, grapefruit juice or apple cider vinegar that comes in contact with your teeth will wear down the protective enamel on them, leaving you far more likely to get a cavity. Simply drinking apple cider vinegar leaves your teeth in a dangerous position, but treating it like mouthwash and intentionally washing the vinegar over all of your teeth is even worse which is what some people suggest.
If you like the benefits of ACV or lemon juice, try only eating them when you are eating seafood drenched in ghee or butter.
Dr. Ringer, a dentist in Anaheim, California recommends these tips for minimizing enamel loss while using ACV :

• Dilute the apple cider vinegar shot in another liquid—preferably water.
• Drink it through a straw so the vinegar doesn’t make contact with your teeth.
• Don’t swish the vinegar around in your mouth.
• Rinse your mouth out with water after you drink the vinegar.
• Don’t brush your teeth immediately after drinking ACV.
• Don’t eat or drink other acidic things—like orange juice, grapefruit, or tomatoes—immediately after ACV.

Personally, I prefer to try to avoid lemon and ACV as much as possible and if I do have some lemon juice I will make sure I consume plenty of calcium rich foods like raw cream or cheese immediately after.  I almost never consume ACV but I find it wonderful as a hair rinse or to rub all over sore muscles before a shower.

I hope this information helps your teeth become stronger and you feel better! Don't give up and EAT BUTTER!

Grains contain anti-nutrients called "phytic acid" or tripsin inhibitors.  Anti-nutrients in grains, nuts, seeds and beans block the absorption of minerals and cause the body to excrete more minerals than it could ever gain by eating those foods.
Agricultural diets produce populations with smaller frames and frail skeletons.  
You'll notice in Holland where they eat a lot of dairy they have large bones and skeletons.
When Weston Price was studying the groups who lived without tooth decay, they all had the ancestral knowledge that grains were dangerous and they need to be properly prepared in order to be consumed.  This would include fresh milling, soaking, sprouting, fermenting and sourdoughing.  These are the steps which eliminate the anti nutrients and turn the indigestible grains into nourishing foods.  I have found however that when you are trying to heal your tooth decay you have to avoid grains all together, even the properly prepared ones.  A traditional, ancestral style Keto diet is the best way to heal the teeth.

In 1937, University of the Witwatersand conducted a study1 comparing crude and refined sugar and cereals and their ability to produce dental decay in the South African Bantu. They found these foods actually created displacement of Vitamin D and thus decalcification of the teeth, as Vitamin D is required for Calcium metabolism. 
The British Dental Journal 193, pages563–568 (2002) published an article showing that the frequency and amount of sugar consumed correlated with increased tooth decay. 
Grains and sugars are really your worst enemies in the fight against tooth decay and the anti nutrients which cause the excretion of your alkaline minerals, creates "the borrowing process".  This process is where your body starts borrowing its own alkaline minerals from the skeleton and teeth to maintain homeostasis in the blood.

1.​journals.sagepub.com/doi/abs/10.1177/00220345370160030201?journalCode=jdrb

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Check out the video below by 'Exclusively NU' reporting on a study which demonstrates sugary cereals cause tooth decay. 

“We can now visualize our universe, its light, gravity and heat, its seasons, tides, and harvest, which prepare a habitation for the universe of vital forms, microscopic and majestic, which fill the oceans and the forests. We have a common denominator for universes within and around each other, our world, our food and our life have potentials so vast that we can only observe directions, not goals. We sense human achievements or ignominious race self-destruction. Every creed today vaguely seeks a utopia; all have visualized a common controlling force or deity as the most potent force in all human affairs. Yes, man’s place is most exalted when he obeys Mother Nature’s laws.”
–Weston A. Price, DDS​