There's a great feature on drugs.com, where you can enter various medications, foods, etc, and check for potential interactions. I inputted the three medications you took, here are the results:
http://www.drugs.com/interactions-check.php?drug_list=703-0,1578-0,896-1617
Hope that helps too.
Oh my, I missed the Benadryl comment all together. PLEASE be careful mixing all of those medications. You definitely need to chat with your doctor. Benadryl isn't usually the best choice for insomnia either. Have you tried Melatonin?
Does the doc who prescribed the Methadone know you take Klonopin too? Methadone and benzos mixed together is typically not recommended, due to the risk of respiratory depression, and even respiratory arrest. That being said, in some cases, docs will be okay with the combination if the doses are small enough, and if the medications aren't taken all of the time, or are taken short term.
This is something you definitely need to discuss with your doctor before mixing them again. Just make sure it's okay. Have you tried other pain medications? Methadone is a bit of a bear, especially when it comes to discontinuing it. I just want you to be aware that there are some considerations with Methadone. Just make sure there aren't better alternatives. For severe chronic pain, I would personally leave Methadone as one of the last possible resorts. Again, just my opinion.
Good luck, update us after you talk to your doc!