Why is it so hard to sleep when I’m extremely tired?

Its a mental issue. You have to slow down mentally before you can relax physically.

Watch ancient history and/or science documentaries. Works every time.

I am not even joking, it really does work, and I also suggested this to my mom, who also cannot believe how easy it is to fall asleep, but we both agree, that we do actually like what we're watching, and will usually go back and try and finish watching it the next evening, and just skip towards where we last remembered, which is usually some show about ancient Minoans, Egypt, Atlantis theories, or like how the universe was formed, but then we always fall asleep half way into watching them, almost no fail... And I think this is because they're usually interesting enough that you want to watch them (like who doesn't want to know about super massive black holes, and if we're actually living in one? I mean, that's pretty deep stuff) but at the same they're usually mundane enough, that you can usually fall asleep after 15 minutes into it, 30 minutes tops, depending on how late it is and how tired you are. Just make sure you are comfy and the lights are off, and have a sleep timer, or don't have it on auto-play. Especially with the Youtube ones, because it just keeps going usually, and sometimes it can play something totally weird later on, and you end up having weird dreams about it, and wake up thinking, why is this video on about pandemic Ebola outbreaks when I was watching a show about Ancient Greece?...

you didn't count enough sheep