I'm sorry to hear you've found yourself in this situation, you deserve so much better than to be somebody's secret.
If she loves you, respects you and wants you in her life then she has to start truly showing it. At the moment she has the best of both worlds - she has a partner who is permanent and as far as you know faithful, loving and hardworking and she has the added thrill of having your love and devotion as well. The question I would ask you is this - if they are no longer in love with one another and acknowledge that their love is simply friendship, why must you be kept a secret? If their relationship is now about financial stability and not about romantic attachment, then surely she should feel safe enough to have a relationship with someone else? If she doesn't have that freedom then it's because her relationship status is not as clear cut as she would have you believe. They are still very much an item, definitely sleeping in the same bed, and the only time she contacts you is when it suits her. Meanwhile you're sitting waiting for 'that' call. If she can lie so easily to a woman who has shared her life for such a long time then it seems only logical that she can and will do the same thing to you. Trust me I've been there - and it didn't end well at all. She's using you because she's bored with her life and wants some excitement - you're the excitement but she likely has no intention of leaving her partner or if she does she'll go back - they always do.
I'm not saying that she doesn't have genuine feelings for you, or that she is lying when she says she wants to be with you. On some level that probably is true, like I say - you're the excitement in her life and are a very attractive proposition as you're there waiting for her. All I'm saying is you really need to let her know that you are not willing to wait around forever while she feeds lies to her financially unstable partner, that's just the latest excuse that she's using to stop you from asking her to leave.
You deserve someone who can give you their love unconditionally, without hesitation, without having to watch over your shoulder in case someone suspects. That's no way to live, sure it can be exciting at first but this isn't a movie, it's real life and in real life you should share an equal partnership. What you're experiencing is not a partnership of equals, it's a manipulation of your feelings to give her a fall back in case things don't work out with her partner. There's nothing noble about her actions, she's not acting in her partner's best interests, she's acting in her own interests.
I know you may not want to hear what I've said, and I apologise if you feel like I've offended you, I'm simply talking from experience. There's someone out there for you - she's beautiful, intelligent, devoted and when the time is right you'll meet her, but you won't meet her until you free yourself from this situation. I wish you luck, I know how hard this journey is going to be for you.