Okay lets get this out of the way, you very likely do not have insomnia.
What you have is terrible sleep hygiene. There is no reason you should be having a nap at 10 PM when you had 7-8 hours of sleep previously. Im willing to bet you spend plenty of time on electronic devices, as well as browse whichever website you enjoy while you're in bed about to go to sleep. You refer caffeinated drink consumption, this also exacerbates the issue. Adding to this, you probably have your lights on right before you go to bed, there might be noises, you might work out very late at night, yada yada. This is all fixable with the right mindset, so thats the good news. Ill let you google "Sleep Hygiene" in order to get a better idea of what you should be doing.
Sleep disorders in general are pretty hard to diagnose, and they might be secundary, a symptom of something else. For example: Depression, ADHD, and other entities can run their course with sleep disturbances. That said, you might be a bearer of delayed sleep phase disorder, which is basically your circadian rhythm is all fucked up. These are the people we know as "night owls". They stay up very late and wake up very late if they dont have school / work. If they sleep at the correct time for them, they have no issue going to sleep / having good sleep / completing their sleep, which is very like you (sleeping 8 hours straight). Something important: THIS IS NOT A DIAGNOSIS. It sounds similar but this cannot be confirmed until you go to a doctor.
Seriously go to a doctor.
I am making such big emphasis because self diagnosing anything is a mistake, but doing it with sleep disorders is really bad. This is because the kind of people to self diagnose them are the kind of people to use a shortcut on the management of the problem. Basically, they skip fixing their sleep hygiene and instead go right to the easy part, taking meds. They read online about how much Xanax to pop a night and since it usually works, they just stay with that. The issue is the kind of medication given for sleep disorders is for short termuse , benzos and company are dangerous if not used right because they can cause addiction and dependence. Suddenly your "insomnia" became a full blown addiction and now youre fucked. There are other issues with self diagnosing but I wont get too indepth.
So yeah, you probably dont have what you self diagnosed yourself with, but GO SEE A DOCTOR IF ITS IMPACTING YOUR QUALITY OF LIFE. They will be able to start a diagnostic algorithm and hopefully reach an answer so you can receive the treatment and management you need.