Some milder things you can do (but may not help) that apparently help.
1. You should remove any sense of time, time creates anxiety and causes you to not sleep. No clocks, try to remove sources of light like windows and what-not, etc.
2. Obvious one but worth mentioning just in case, don't go on your phone or electronic device, even on night mode the light from screens can keep you up.
3. If you find yourself just lying in bed for a long time, move somewhere else and lie there, and then go back to your bed when you're feeling tired. The idea behind this is you start associating your bed as a place for good sleep rather than a place you stay up for, for a long time.
Melatonin only has a very mild effect and the effect is only really noticeable for older people so it often won't help. Benadryl is a cough medicine and probably shouldn't be used for sleep. Check to see if the medication you're on doesn't have sleeplessness as a possible side effect. I would not recommend taking sleeping pills (even over the counter ones) cause they're quick to cause dependence, although when I was having great insomnia (wasn't able to sleep for days, and when I did sleep it was only a few hours) I took them for important events only. Still wouldn't recommend it.
If your sleep is caused by anxiety it isn't a bad thing to seek mental help, mental health shouldn't be a stigmatized thing, treat it simply as a potential means to get what you want, which is a good night sleep. Doing a sleep study as vosur said can also be effective.