Cardiophobia won't let me focus on anything

I will try my best to keep it short

So I'm 19yrs old and last year i finally became a Nihilist from a religious guy who would blindly and strongly believe in god . From debating with atheists all the time and defending my religion to literally turning into an atheist is still unbelievable to me .

Now from couple of months I realised that the only promised thing in life is death and one day i will die too . Became thanatophobic and soon cardiophobic . Would think of death for almost all the time when I'm alone .

Now last week I was just chillin when Outta nowhere my heart started pounding, I felt strong sinking feeling around my heart , adrenaline rush and i thought I'm dying (i sometimes used to feel palpitations but only when I think about death, this was first time it just happens randomly) I panicked so bad 😭 even accepted that it's time to go .

Went to the doctor next day and I was feeling like this for consecutive 3-4 days Now i got my ECG , holter and other blood tests done and all the doctors including cardiologist said it's cause of stress and anxiety and i don't have any heart issues. I realised it was most prolly the panic attacks But since then i can't even focus on my studies Even today in the morning i felt all that again and I was like I'm going to die again today

Now that I don't even believe in god I feel so hopeless and I'm scared of getting a cardiac arrest . I don't even sit in my room alone thinking I would die and there wouldn't be anybody around to help me . So I just try to be around someone all the time . 😭😭

What shall I do ?