Oh boy....
If she has trust issues which are not of your doing she needs to get these sorted out before you get hitched...
Whether or not you take part in that process is your choice, is the relationship that strong and with such a good future?
Only you know.
But from what you have said its part of her baggage she is bringing not a result of your relationship. On a selfish level you could say why should I suffer the effects of somebodyelse's actions.
If you do deciede to go forward with the marriage and the 'conseling' you have to be firm in maintaining your postion as somebody who is there to help your chosen partner to come to grips with issues, you are not the cause of the problems, nor should you be expected to change your life to accommodate somebodyelse. So do not allow any of this 'you must understand, you must accept'..lines from counsellors.
If you do get married or even live together then both parties have to change to accommodate one another, and its a bit hit and miss. No plan survives contact with the enemy
No I'm not married but have been living 'in sin' for a number of years and our son is a cheekly little so and so. Getting married or being married is not an issue as far as I know with my better half.
And who wants to be like everyone else (at least in our families..)
Wishing you good judgement in your choice.