How to maintain a Buddhist practice through dark times?

Buddhism has slowly entered my life since the last 25 years. Though born a Hindu in India, I have always been open to practices that would help me answer my fundamental question about the meaning of life and, more specifically, how to attain the end of suffering and final release. I followed many traditions but none satisfied me. Meanwhile Buddhism, specifically Theravada Buddhism, sort of grew into me owing to its very scientific and rational approach to liberation, to Nibbana, sans gods and the like. For the last year or so, I have been intensively doing Samatha Meditation, Anapanasati specifically, and have been putting ~4 hrs on workdays and 6- 10 hours on weekends in sessions of one to one and a half hours. Initially, I went through a lot of physical problems, mainly severe flu-like symptoms, but I persisted and these have thankfully subsided now. At the moment, however, I am feeling very downcast and it appears that I am lost. There seems to be darkness all around, yet I am persisting with my intensive practice. These 'dark nights' make me so despondent sometimes that I feel that I may be having too much of bad karma that is obstructing my growth. My only motivation is Nibbana, nothing else, and I would go to any length to reach it but then something is not allowing me to move forward. Some time back, before the pandemic started, I was planning to go to Myanmar for a long retreat but CoVid- 19 simply poured cold water over my plans. Could someone guide me in this hour of distress?