Is it normal to sleep 15 hours

This is normal on occasion when you are exhausted or sick.

It is not normal long term. It indicates something is wrong, and you should be looking for what is wrong.

I once slept something like 36 hours after a week-long extreme camping experience as a teenager; and often slept for 20 hours a day for 2 days after several weeks of power plant outages as an adult (and so did the other workers). Those are cases from exhaustion.

When I had sepsis I typically slept 16+ hours a day in the hospital until they got the infection under control. That's an illness example (but a bad cold or flu case will do the same).

I wish you the best with that,

That's a gift.

No, no it is not.

It may be a sign of depression.

Sometimes it’s just one of those things, it usually takes me 5-8 hours to get to sleep every night and when I saw a doctor about it they basically just told me to get over it

That's a nap to me.