Will Allah not answer my supplication because I have broken my covenant with Him by violating my promise not to commit a specific sin, and how can I atone for my repeated violation of a covenant I made with Allah, and does voluntary fasting expiate sins and transgressions?

Whoever repents, Allah will accept his repentance. So, no matter how great the sin or how often it is repeated, let the servant repent with sincere repentance; for the one who repents from sin is like one who has no sin. He should also supplicate Allah Almighty, have a good opinion of Him, and do many good deeds; for indeed, good deeds efface evil deeds. As for what he covenanted with Allah and then broke his covenant and went back on his word, he must offer expiation of an oath for each of these covenants.