*Disclaimer: You will NEVER be "rich" solely on a military salary.
1. I was enlisted for 7.5 years; I have always lived "comfortably." It can be done. The key is not to be "stuck on stupid."
2. I am married now, my wife is in grad school (not working), and we are still living "comfortably." In fact, we are living so comfortably, that this year she has attended grad school full-time, we each bought motorcycles, I bought a new truck, we both maxed out our IRAs, I maxed out my TSP, went on a few vacations, and we still accumulated sizeable savings.
Here are a few of our "secrets":
-She clips coupons, and we shop at the commissary as much as possible. It is almost a game to her to see how much she can save each week.
-We go out to eat occasionally, but not every night. You'd be amazed at how much money you piss away when you dine out every day. Also, I brown-bag during the day as much as possible.
-We discuss and define our "needs" and our "wants." Did we need motorcycles? Hell no. In order to make that happen, we did away with some of our prioritized our "wants."
-We budget for today's money, not for what I might make in "X" months due to raises, flight pay, etc. While the military's pay process has gotten a hell of a lot better in the last decade, it is still a good idea not to have 100% confidence that all pay/allowances will kick in when they are supposed to. Also, who knows what the future will bring? What happens if I get into a car wreck tonight, get badly disabled, and get discharged soon? What happens if my orange-and-white bird becomes a smoking hole? Life insurance is good stuff, but it should not be an excuse for living outside one's means.
3. Do not let her opinions determine your future. Don't want to be an ass (though I'm sure I'll piss off the Spouse's Corner), but she is your girlfriend. She is not your wife (if you were married, your finanical situation would be a much bigger deal), or even your fiance. If she decides that she can't live "comfortably" (based on her definition) what are you going to do? Not join? Or will you decide that maybe she's not "the one?" Don't put yourself in a position to make you kick yourself for the rest of your life for making/not making a decision as important as this one.
4. DO NOT become financially irresponsible (i.e., live outside of your means) as an officer.
I hope some of this helps.