That sucks. I'd echo the other comments that say this is salvageable, but priority has to be a) you protecting yourself and b) her being able and willing to get the help she needs to address whatever lead to this behavior. Both the excessive spending but also hiding it and going deep into debt on your shared account.
If you are in a relationship with someone who has a destructive addiction then you are exposing yourself to the fallout from that.
Also I'm sorry to hear all this, obviously this means a lot given that you are engaged and she helped you in the past.