Sleep problems

I’ve been struggling with sleep since I was in high school. I’m almost 21 now so it’s been a long time having to deal with this. My main problem is the excessive sleepiness during the day. No matter how hard I try I cannot stay awake during the day. At night, even if I have forced myself to stay awake all day, I can’t seem to fall asleep at night. However it isn’t like this all the time and I can fall asleep at night sometimes. In the car or if I am sitting for long periods of time, I have definitely nodded off and fallen asleep for a few mins. Another thing I have been struggling with is constant nightmares. Not scary or unrealistic nightmares either, my nightmares always include people I know and they are scarily realistic situations. I have dealt with some traumatic events in the past couple years so I thought maybe my PTSD is making things worse. I also have had depression for years (specifically Dysthymia or chronic depression). I have tried so many times to correct my sleep schedule but no matter what I do I still get so tired during the day. I know I have to ask my doctor about it but I wanted some opinions before I go.

Definitely get your doctor's opinion. If you haven't had a thorough physical with chemistry lab work, that may show some serum numbers skewed.
Rule out any physical things, like high or low test numbers first. Have your doctor refer you on to a specialist, if necessary.

You might consider a sleep study where you are monitored during sleep phases for anomalies.
Sugar, carbs, salt, tobacco and alcohol, etc before bedtime can affect sleep.
Complete darkness, silence and deep breathing before drifting off to slumber helps me. Intense dreams can leave me shaken the first part of the next day as well. I had some very realistic dreams when I was undergoing some major life changes. They aren't as disturbing now as they once were.
Best of luck to you.