Speak to your pdoc about adding a mood stabiliser - if you don't experience huge highs, the lows may not seem so bad??
I feel that 40mg of Lexapro is quite a strong and if you have been on it for a considerable amount of a time (A month or longer) it should have helped if it ever was going to. Otherwise can I please recommend you go and speak to your pdoc ASAP and get him/her to try you on a different anti-depressant. There really are so many different ones, and while it can be frustrating, it may take a while to find the right one for you. I for one did not get on with Lexapro. (Or it with me :-) )

Has the depression got worse over time? Can you attribute it to any triggers?

Please ask your pdoc for help - severe depression is very debilitating and strains every relationship in your life. You also deserve to be healthy.
Hang in there - we are here to offer you encouragement and support