Is there still repentance for me after committing many sins and grave transgressions, or am I now considered an infidel?

You must engage in sincere repentance and strive for uprightness according to the Sharia. Indeed, Allah’s mercy is vast, and He accepts the repentance of His servant if it is sincere, forgiving all sins. Turn away from these whispers (waswas) and strive against your soul to get rid of them, for as long as you dislike them, they will not harm you. Be diligent in performing Fajr prayer in the mosque. If you pray in congregation at home, your obligation is fulfilled. Repent to Allah for your statement, "I challenge Allah to heal me," and do not repeat such a statement. Do not find prayer burdensome; rather, be keen on performing it with humility, and stay away from bad company.