How can I get her to let me cum in her mouth?

She point blank refuses so how can i convince her to try it?

I absolutely think that this is one of the most intimate things you can do, and you need to compromise with her on this one, its a tricky one. Try this::
Get her to go down on you, but let her know when you're close, (dont just blow your load in her chops, she'll be far from happy)
so she has the decision whether to pull away or not. Let her take her time, both of you should enjoy the experience , you for receiving, but she has to enjoy the giving. Why dont you ask her let you cum a little bit near her face, if she refuses to even do that, forget the whole thing because it'll start a fight, and you'll both be upset. Just respect her decision that she doesnt like it.
Hope that helps you