The only way to stop is by choosing to do so. No one is making you get married.

I think the best thing may be to put the engagement on hold and if you're willing, initiate or carry on with a therapy that best works for you. This could just be who you but perhaps your actions are being exacerbated by your condition. It's really not that uncommon in people with PD's. Therapy may not have any bearing on what you do outside of treatment - after all, you're still responsible for your own choices, but it should certainly help you to consider things with greater perspective and focus if nothing else.

Not everyone suits marriage. And this is perfectly ok. If it's still something you feel you want, maybe taking a break from the idea and working on improving how you feel is the best way to ensure greater success in future relationships. I understand the pressure you might be feeling, but you said it yourself - you don't want to be going through this at your age especially after two other failed marriages.

Good luck.