I’m sure you’ve heard it before, but you can’t change other people. We all know it, but we tend to act as if it isn’t true, or we convince ourselves this one situation is different.
First and foremost, you must accept that a person can only change by their own decision and action to do so. Furthermore, any efforts to push them in that direction are most likely to cause the opposite reaction.
Do you two live together now? There’s no need to jump into marriage. I have a friend who has been with his high-school sweetheart his entire adulthood, they even raised a child together. They just got married this year.
The best way to know if a marriage will work, is to live together. If you already are living together, understand that it doesn’t change just because you put a certificate on it. What you have living together is what you have after marriage as well.
Control what you can control: yourself. Do what you need to for your own treatment first! Don’t let the relationship hold you back from that, because if you do it always will. If your efforts in that direction put a strain on your relationship, then that’s why your current therapist recommended not getting married. By working on yourself in spite of it, you will either learn the relationship is not going to work, or you will inspire her to do so for herself as well. But don’t do it for that reason, do it for you with no expectation of her following.
Write down all the things you’re willing to do and put up with under the assumption that she will remain as she is now (not how you’d like her to be). Do you still feel like you must get married, even under this assumption?
Then spend some serious time thinking about whether or not you can spend the next 5 years, 10 years, 15 years, etc. under those conditions. How will life’s inevitable challenges change things?