need support for lexapro

I'm at the moment on lexapro . I started taking it about 4 weeks ago. I was on 10mg for 2 weeks and for the past 2 weeks I been taking 20mg.
I was diagnosed with bipolar at age 21 and i'm 30 now. I've switched Doctors recently and my new Dr. Decided to try only an antidepressant this time. She wants to see if I really need a mood stabilizer.I have always been a little curious if that Dr . I had back at age 21 diagnosed me correctly .

I'm writing this thread because I'm pretty worried that lexapro is not going to do anything for me.
It's been 4 weeks and I have had a hard time staying asleep at night . I fall asleep fine.
I also find my self feeling a little heady sometimes. For the past 2 days i've noticed if I use lots of energy I get a little light headed too. I keep reading that it takes awhile for lexapro to start to work like it's suppose too. I still feel depressed and find my self thinking a lot.

Should I stick with this medicine for a little longer?