As I have said, you really need to educate yourself on bipolar disorder, specifically on your fiance's symptomology. Talk to her mother about how she acts out. You know she self-harms, you know she withdraws and questions your relationship, what else does she do? Re-read Ohlala's post for other very commom ways bipolars behave when they are cycling.

Then you have to ask yourself, can you and your relationship survive these trials and tribulations? Look at how much you are struggling now, can you handle a lifetime of this? If you are in this for good, I already gave you suggestions on how to assist her in another post.

When you say "I feel as though doing too little will mean she leaves or she makes a mistake as I haven't helped her enough, but at the same time trying to bug her might come across as desperate and may drive her away." it sounds as if you are trying to control the outcome of her decision. All you can do is offer your support. Tell her you understand she is struggling right now and that she doesnt have to make a decision until she is better. then yes, just sit and wait it out.