There are three Ashra's (parts) of Ramazan out of which the first Ashra is for magfirat ( forgiveness ).
So use this Ashra to the fullest and repent from your bad deeds and ask for forgiveness to the almighty and promise him to not commit the bad deeds again.
Also stick to your promise. The days have already passed and the fourth has already started. I hope Allah S.W.T forgives Everyone in the first Ashra.
Aameen.