I think it really depends on your attitude. It sounds like you would want to make it work with your girlfriend though, so that is a good start. The main thing is communication. It is really difficult for OHs to understand the time, level of study and pressure involved in getting licences, and later, with working as an airline pilot. You need to sit down with her, talk it through and be brutally honest. She will need to be sure she can support you 100% and she needs to understand that you will have a one track mind during much of your training!!