When I first started taking Lexapro I had a bit of a problem getting to sleep as well. My doctor initially put me on Ambien and after a few weeks I switched to benadryl (an OTC antihistamine which is also the sleep aid ingredient in the "PM" versions of many pain relivers). Over the course of a couple of months I was gradually able to stop taking the benadryl to get to sleep. I've been on Lexapro for about 3 years now and it's improved my quality of life significantly. I gained some weight over that time but that could be due as much to my crummy eating habits and generally getting older.

You would of course have to check first with your MD or pharmacist to make sure there are no contraindications w/ the Topamax, but you might want to consider taking benadryl as a sleep aid for a little while until you've adjusted to the Lexapro.