Again, stop saying it's this big 'lie' and that she 'betrayed' you. I mean, you said you have wanted her to see a therapist for years and never told her. Isn't that kind of a betrayal? As for her not being honest with you, sometimes therapy is something people want to keep private so focus on this being a chance to be more open with each other and how great it is to want to grow and seek help.

If you want to seek therapy, please do it for yourself, there are often some free outlets for you to at least get an initial few sessions like through workplace benefits, etc. But don't hold it against your gf that she has done this for herself...you can't resent her for doing something you wish you could, but haven't.