Welcome, both of you, to B2W ... and I think Felicia gave DMWV good advice.
...DMVA, the first thing I do before commenting "with advice" is to look at the person's profile, and even some of their past posts to give me an idea of their maturity, ages. experience, etc. You haven't filled that info in as of yet, so, I encourage you to complete your profile IF you are going to ask for advice from forum members. Some of us actually take questions seriously and try to give good advice, but it helps to have a little info on the subject. For all we know you and your GF are teenagers or early 20s.
...If you haven't developed your relationship to a point to where you can discuss sensitive matters with your partner, the next best is to make it a "fantasy sharing" type event ... where, if she feels offended, you can laugh it off as just fantasy even if you are serious. You say you've been with her for years, yet, you may not know your partner as well as you think you do ... possibly, if you've been together for 5 years, for example, you have 5 one-year experiences with her and haven't really learned a lot about her. Maybe you'll find that you've been dating each other for convenience, not for building the relationship.
...An easier way to open the sensitive topics, like this, is to simply make it a "sharing" topic ... like "let's talk about our sexual fantasies" or "what your deepest sexual fantasies are". You may find her sexual fantasies the same as yours, but most likely her fantasies will be somewhat different. What it does give YOU, however, is the opening to share your sexual fantasies with her. The ladies tend to be quite reluctant with sharing these kind of "suppressed thoughts", but if you can have some good drinks, like margaritas or a good bottle of wine, she's likely to drop her "sharing personal info" shield long enough to give you some ideas where her sexual fantasies lie. Then, you set about having some fantasy sex play involving those deep fantasies, and eventually make them become real.
...But, if you're concerned about opening up such topics, you guys are not growing as a couple ... simply milling away the time in a relationship that's not maturing, and that would be a waste of time. Relationships get more interesting as time passes ... I know, I've been married 21 years as of this September.
...Hopefully, more forum members will take your question seriously and try to help you. Ask more questions if you like ... this is a very sensitive subject, I know.
...Best of luck ...
...Mac