You are overbrushing your teeth. It had caused wearing off of teeth.
There might be highly calcified tartar near gums and not decay. Clinical examination required to know if decay really present.
Visit dentist for clinical examination and ultrasonic scaling. Scaling is completely safe and painless procedure.
Kindly go through the blogs mentioned in the links below.
Next Steps
"10 Dental Commandments for Happy Teeth!" by Dr.Rashmi Bamane - https://www.practo.com/healthfeed/10-dental-commandments-for-happy-teeth-40426/post
Health Tips
"Side Effects of Wrong Tooth Brushing" by Dr.Rashmi Bamane - https://www.practo.com/healthfeed/side-effects-of-wrong-tooth-brushing-40067/post