What sort of incompatabilities? I'm trying imagine what they could be and why they would not come out during a long courtship.
You'll learn soon enough how different your relationship is now than it will be when you are married. Not better or worse, certainly, just different. Right now you only see each other on free time.... free time and vacation. People are different when they are on recreation time than they are when it's cleaning, doing chores, and getting ready for work time. You're different at home doing laundry than you are out with P, aren't you? Living together, married or not, means you're bringing someone else into real life, when you really become the full range of who you are, and you'll meet the full range of who P is... And it will be different than the best behavior of courting and play time.
The same sort of incompatabilities that break up marriage. In courtship you usually see each other on their best behavior. People make an effort to put on their best face. People are going to let go a little, however, if you're living together 24/7, you won't be constantly attentive to each other when you're together all the time (it's usually not that way when on a date), and the differences you each have will be more in your face. Basically, your negative qualities (in terms of personality, lifestyle, and attitudes) are much more likely to show themselves.
Lots of stuff that could ruin a marriage. E.g. he always goes tot eh bathroom in the middle of the night, she doesn't like football, etc.
/sarcasm