That's what everyone says about purchasing bitcoin or other things that are worth much more than before but this feeling of regret won't pay you anything, the only thing you get when you feel regret of not investing sooner is a bad feeling and this bad feeling will effect you future disicions in the market. Obeseouly if you or anyone else investing in bitcoin in the early years you could become rich with out of doing anything.
But instead of feeling regret you should take session from the past and try to use the chances market gives you in the future it was not the only chance we got so still you can use the chance next time.