Hi there. Yes, anxiety can impact your digestive system. And hence, your elimination and constipation.
I think you need to speak to someone about medication. If you tried just ONE Lexapro, this doesn't give it a chance to work. I would wonder at this point if your anxiety didn't make you ill the day you took the Lexapro. Don't let anxiety win. Put it in your place. Think of it in third person and stand up to it. Medication will put it in its place along with talk therapy. It's something invading your life--- do what you can to extricate it.
Go back to your doctor, discuss what happened with your Lexapro and come up with a plan. Then stick to your plan. Fight through any physical symptoms when you first start a medication as they truly may be anxiety driven rather than the medication itself.
Again, don't let anxiety win! You deserve a peaceful life without having that beast try to ruin it! good luck to you