Doxylamine and Diphenhydramine belong to same class of drugs called anti histamines and are available as over the counter sedatives. If they are ineffective you may have to try other sedatives after consulting your doctor. Most of these sedatives have addictive properties and have to be taken carefully as per prescription.
I would recommend a short acting sedative like zolpidem 10mg. It is a non-benzodiazepine and has less addictive properties and side effects compared to conventional sedatives like benzodiazepines. Although less addictive it can develop dependence and can be ineffective when used for long periods. Anti-depressants like Amitriptyline can be used for short term sleeplessness but anti-depressants usually have withdrawal effects like benzodiazepines and dosage should be tapered slowly. Remelteon, a newer drug is very effective in treating acute insomnia. It is a Melatonin receptor agonist which acts directly on sleep center and thus does not have side effects or abuse liability like other sedatives.
All of these deatives are prescription based and have to be taken only after consulting your physician. It is also important to identify the root cause for your lack of sleep. Insomnia can occur due to mental stress, depression etc. Without addressing these issues, drugs alone won't be effective. If your problem continues, I would suggest you to undergo detailed evaluation. Tests like polysomnography can help in understanding your sleep pattern. Cognitive behavior therapy along with medications will be more effective in treating insomnia.
Hope this helps. Thank you.