Thyroid can be an issue here if the problem has been going on for a long time. There are some other signs for it, and generally, we aren't doctors, and you could totally get it checked in case you are worried.
However, there are some other things going on that might be very much use to you.
First, for your sleep habits, you could check out "Sleep Hygiene." It's basically a great way to help yourself get tired, and keep a routine, so that you are tired when you need to sleep. I follow this, and if I don't, my sleep schedule slips really far out of whack. I don't look at screens for half an hour before bed, dim the lights, no caffiene- well, for me, none at all, because it gives me migraines. I go to bed literally at the same time, as close as possible, every single night. Like, I don't care if I work, or if I'm not, or whatever is happening, same time, every day. People want to hang out late? Oh, no, sorry, I can't. You wanna go for a Timmies run at 11pm? Nope, I'm already in bed.
So, that's helpful. I also find that napping can be very destructive to my delicate sleep balance, so I try to avoid them (though, when I have my monthly cycle, I tend to sleep a lot more, so naps are acceptable at this time).
Other than that, I find that when I get up in the morning, I am also fairly motivated to get things done. By the evening, it's more like I'm wanting to rest, I've spend my energy, and I'm starting to wind down for the evening. I don't get depressed, exactly, but I get more relaxed and a bit tired.
It sounds like you deal with a bit of depression in the evening, and I've heard of this happening to people at a certain time of day.
Perhaps, if you can, find a way to change your routine at this time. Right before you hit that depressed point, see if you can get some exercise going, or do something lovely for yourself.
Also, as this is a pattern, you can look at the patterns of your life, as well. Is there something that happens each day that you feel unhappy about? Do you get depressed consistently on certain days, and not other days? What happens on the days you don't get like that (or, what doesn't happen)?
Don't be afraid to ask questions, and try to change things up.
It's your life, and you deserve to be happy with it. Change can be really scary, but in the end, you'll learn a lot, and you might find you are ultimately much, much happier afterward.
