James,
The hardest things are going to be whilst you are studying, as you will need to be very selfish and focused during this time.
Then you will be broke for quite a while until you are well into your first job and able to pay debts back.
Then it depends on the type of flying you are doing. Excluding BA, if you are flying short haul you are generally back most nights, and will fly a few nights a month as well. If you are long haul then you will be away much more, with more opportunities for straying. If working for BA I think its a bit different, as the short haul fleets do seem to do more tours of a few days at a time, although they probably get more days off in a month too.
I think that the most important quality your girlfriend will need is to have a full life herself, with plenty of interests and friends and basically not mind her own company and doing her own thing. You can probably work that out yourself - do you spend all your spare time together, does she demand all your attention?
It is easy for someone to say that they will support you, but unless they are their own person, they may put you under additional undue stress just when you don't need it.