Contemplating how to ask a girlfriend of 2 years how to take it to the next step when her view has been soured by a late parental divorce.
I was just at a friend's wedding last weekend, am engaged myself, and four other friend pairs are also engaged. In all of these cases, we discussed marriage extensively with our partners before anyone was officially asked. We all knew what the answer was going to be.
If you want to consider going to the next step, you should bring up marriage casually in conversation. If she sounds like she's just not into it, she's either going to say no, or say yes because of pressure when she doesn't really mean it.
I recently asked my very excellent girlfriend of 5 years said question. I procrastinated so long at least partially due to residual trauma from an early parental divorce (plus the subsequent decades of court battles and screaming).
My fiancee's approach was a combination of patience and understanding for 4 years, then basically telling me my time was up. The latter strategy wouldn't have worked without the former.
One thing I'm certainly glad we did is to talk about it honestly and deeply before taking the plunge. I doubt I would have been able to work up the courage to do it otherwise.
If marriage is important to you, tell her so, and give you reasons why.