Hi Judojamesg,
I have worked for four airlines and I haven't spent a great time away from my family. I was there the first day my son walked, talked, laughed etc. I also know that I will not miss a birthday or any event for which I can give my company notice.
Whether your relationship will survive a Commecial pilots course, times of difficulty ( like looking for your first job ) Initially a poor salary ( hopefully not) and so on, is entirely up to the individuals concerned. However, the fact that you are asking about these issues suggests to me that they will not be a problem. The other fact of the job is that when you do have to spend time away there is obviously a trust issue that needs to be addressed, and that is only something that you and your girlfriend will know about, and that is not just aviation linked.
What I am trying to say is that not all flying jobs mean you spend time away from home. Once you have a little experience you can choose who you want to work for, and then you can balance your life so that you can still do a job you love and have a family life. I know I see more of my wife and son than most of my friends with so called 9 to 5 jobs.
I wish you well in your decision making but pesonally I'd go for it.
Regards Le loup