I recommend seeing this video as it can potentially change the way prayer impacts you:
Also, keep yourself super busy so you don’t have time and you don’t have an opportunity to sin.
Don’t convince yourself that your heart is sealed. Satan wants you to lose hope. Do istighfaar 70 times a day and say la ilaha illa Allah 100 times a day (when you say them say them slowly and take a deep breath) and Insha’Allah your heart will open. Allah created your heart. Don’t let anything take your heart away from Him.