Why is my boyfriend always frustrated and impatient with me lately? He says he loves me, but then acts irritated by everything I say or do. 11 months together, and this is somewhat recent and getting worse. He is also 4 years divorced with children. I have never been married or had children, yet I treat his like my own. We are both 35.

If you have to ask total strangers, who can't possibly know which of thousands of reasons he might have, then I have a feeling that you aren't a good communicator. It's time to find out from him!
Do it in a way that doesn't blame. Say 'I feel confused by mixed messages lately. It seems that you are irritated with me, despite saying you love me.' Find a good quiet calm moment to bring it up.
Just from your ages and his previous marriage, is there any chance that you have been hoping for marriage and he senses it (or you bring it up) and he is far from ready? Have you two talked about the all important matter of you having children? Do you want any? Does he? If your clock is ticking, you can't rely on hope, you have to be practical and out in the open about it.

Sorry you have to ask the irritated, frustrated angry guy what's the bug up his butt!