If this is a problem in your relationship right now, what do you think it will be like in 10 or 15 years time?
You have had the experience of marriage one time in your life and that has taught you quite a bit. Are you going to use that knowledge to make future decisions, or are you going to ignore it and hope that everything fixes itself with time and patience?
You may not want to hurt your girlfriend, which I believe With all sincerity, however it will hurt her more for you to stick your head in the sand and stay with her long-term only to find out that you have grown to resent her enormously for not giving you enthusiastic sex for years. You will no doubt come back here to tell us that nothing has changed, and in fact, it may have gotten worse.
Your gut is already telling you that this may be a big problem. Provided you have communicated to her very clearly about what your expectations are for intimacy, then she should be as equally truthful about whether she can meet your needs or is unable to do so. Also, her actions will speak volumes over her words.
You should take into consideration the possibility that she is very happy with the way your relationship is as it stands and she sees nothing as lacking. It may not be fair to you, but it may be her reality.
Given that you've been to sex therapy, both as a couple and her on her own, and have seen no results or change, indicates that this as good as it's going to get. And I have a feeling that it's only
this good now because you're not married yet; if you do marry her, there's a good chance that the sex will dry up altogether.
I have to give her credit. She's trying, which is more than a lot of people get from their partners. But sometimes, simply trying isn't enough, if one doesn't succeed in making actual change and improvement.
It's not her fault, and it's not your fault, but this relationship isn't working on one very important facet. You need physical intimacy with a willing, enthusiastic partner (that's what makes it truly intimate!), and despite her efforts, she is unable to give that to you. She wants to, clearly, but she simply cannot. Sadly, I don't think that she will be able to understand exactly how important this is to you, and WHY it is so important to you. (Those of us who are HD and are/were stuck in sexless marriages understand, but none of us have ever been successful in getting our LD spouses to understand this.)
You don't want to hurt her, and while that's admirable, it will be unavoidable if you honor and prioritize your own needs... because you will never have your needs met fully in this relationship. She sounds like a wonderful woman, but if you stay with her--if you marry her--you are facing a lifetime of struggle and disappointment in terms of physical intimacy. Is everything else about her worth giving up the physical intimacy that you know you need?
It's going to be hard, because she WILL take this personally, even though she already knows this is a problem. She will feel hurt, especially if she's expecting a proposal (not to mention the baggage with the ex who wasn't willing to put a ring on it after SEVEN YEARS). But is her happiness more important than yours? Is a relationship really happy if one person is happy and the other is not?
She's a great woman, and she's perfect for someone... but she's not right for you, as amazing as she is. You both deserve to find someone who is a better fit in terms of sexual compatibility.