Is what I feel after being unable to abandon a sin that I made a covenant with Allah to forsake a sealing of the heart, and is the sealing of the heart a good thing?

Disobedience causes darkness in the heart and tightness in the chest, but Allah forgives all sins. Do not despair of Allah's mercy, and repent to Him now. If you sin after repenting, renew your repentance, for Allah forgives sin as long as the servant knows that he has a Lord who forgives sins and holds him accountable for them.