I need to preface this with a disclaimer -- my first serious startup (which was very successful from a business point of view) cost me my marriage. The reason was that I worked 16 hours a day, 7 days a week, for about 5 years. Very few relationships can withstand that level of neglect.

So, my advice is coming from failure, not success. I can't claim that I know how to do this right, but I've taken away some lessons from that failure, and I have subsequently been able to start and run businesses without damaging my relationships.

First, kudos to you for facing up to this issue right up front. That's more important than you may think. And you're 100% right: your girlfriend -- like literally everybody -- needs to feel valued and important. If you consider that relationship to be important, she really does need to come first! Your business should come second. Do not lose sight of that order of priority.

Your business should be the close second because taking care of your personal life cannot be separated from taking care of your business. If your personal life is struggling then that will inevitably have an adverse impact on your performance at the startup. Also, it's very rare that the business actually has to come first in terms of where you spend your time and energy (as in, making it a close second is unlikely to make a meaningful difference to your business -- but it is very likely to make a meaningful difference to your significant other).

What I do these days is to prioritize a block of time that is exclusively to be used with friends and family -- and especially my significant other. This time doesn't have to be a large percentage, but it has to be inviolable. No matter what else is happening, that time is devoted to my loved ones.

Carving out time for her and her alone (even if it's only an evening per week) tangibly demonstrates that you value her and consider her important. I think that you may also find that the more you demonstrate her importance to you through action, the more she'll be willing to give to help you in your business activities. You may be surprised at how much crap your significant other is willing to put up with so long as she truly knows that she's more important.

Also, (if this is a very serious relationship) she needs to be your partner in your business activities just as with the rest of your life. Not necessarily in a legal sense, but when business-related decisions have to be made that will impact her, you should include her in the decision-making. Act as a team. This will likely give her a greater sense of security.

In the end, try imagining what it would be like to be in her shoes. How would you feel if she were the one in the startup and you were the one affected by it? What would increase your comfort and happiness? How would you like her to behave? The odds are that's pretty close to how she feels. And don't forget that she'll be sacrificing things for you in this endeavor. Make sure that those sacrifices are not a loss to her, but rather are an investment that she'll get returns on.

EDIT: I feel I should add another thing... you should communicate fully, honestly, and continually with her. Tell her what's happening with the business. When things look dire (and they will), resist the urge to hide it from her. When things are looking great (and they will), share that to the same degree. When people have to guess what's going on, they will tend to imagine things that are far worse than the reality -- so make sure she always knows the reality.