Pray to Hashem for the clarity and wisdom to get through this difficult time in your life. You stated that you’re not an atheist, but that you’re not sure you believe in Torah miSinai and therefore have trouble doing teshuvah for aveiros that “may not exist”. So focus on teshuvah for bein adam l’chaveiro, as WolfishMusings suggested, and as for bein adam l’makom, focus on building or rebuilding your relationship with Hashem. Ask him to send you the guidance and support you truly need. He is your Father. Just be honest with him and daven to Him in your own words if you can’t connect with the nusach.
Much hatzlacha. I will be thinking of you and hope you emerge from this trial stronger than before. Don’t let the haters get you down.