[Dr. J Chakma]

Hi Saket
With my little understanding of Buddha Dhamma from this website (I have been born Buddhist, but hardly had any true idea about what really is Buddhism), here is my explanation to your question/doubt.
You (and we all, the sentient beings) have been born and exposed to Buddha and Buddha Dhamma, infinite number of times, even we all got exposed to true Tilakkhana. However, none of us, who are still in sansara could attain Nibbana, because whenever we were born during Buddha Sasana or even during time when Buddha was alive, because
(a) we all (including you) most likely had dihetuka birth and could not grasp the meaning of Talakkhana and we did not put enough effort to change our gathi to be born tihetuka in next life.
(b) When we had Tihetuka birth and was also exposed to pure Buddha Dhamma and true Tilakkhana, we did not put adequate effort in this.
I have a small advice too that might be beneficial. You should not worry about this problem and concentrate on niramisha sukha or cooling the mind and getting to peace of mind and give your best possible effort in practicing and learning pure Dhamma.