Debbie,
I agree, it's just part of the "normal" icky part of that last trimester. The only thing that I found helped was exercise. It doesn't have to be strenuous, but I found the more I sat on my butt with the other kids - because I felt big and uncomfortable, the worse the insomnia was. For me, physical activity helped fatigue me physically, getting air seemed to help me mentally and then I was ready for sleep. I realize it's a bit more challenging for you since the weather is turning, but go mall walking, push a stroller, push the kids on the swing, ANYTHING to get YOU moving and exerting yourself A BIT. Don't simply casual walk, try to pick up the pace a bit.
Be careful artifically sedating yourself. Benadryl is very dehydrating. I know you crave the sleep, especially chasing another child during the day, but God has a plan. You never truly know the side effects (no matter what doctors say). I don't think anyone would recommend taking benadryl as a sedative in your third trimester anyway.
Just know that the end is near. Try to go with the flow as best you can. I can remind you - since I have a two month old - those first 6-8 weeks are rough. Mixed up days and nights, lots of night wakings, etc. God really is just trying to prep you now for life in January.
Try to get naps/rest in when you can during the day. Learn tricks to get your other child to "mellow out" with you when you can. Think of this as your "opportunity" to learn ways to teach your #1 child to be patient, play alone, etc.
Good luck! I hope all goes well with your pregnancy and delivery.
Sara