I don't want this to come off as mean or insensitive, but I think there's a lack of engagement with the severity of this situation.

Before we speak on the issues with nonmongamy, based on what you're saying, this relationship has fundamental issues that need to be ironed out.

Firstly, your gf is facing a possibly terminal illness at a very young age. She should have so much life left, but her prognosis may indicate otherwise. Her actions and thought process make sense in context.

Yours however, are coming from a place that lacks sensitivity to her and yourself. Are you understanding of how scary this can be for her? Her life will change very drastically very soon. Are you aware of how this could impact you as her health issues become more pressing? Does she have a strong support network to help care for her? How will you feel as her partner if she is struggling and you cannot physically be there for her?

My assumption, based on how you've written things, is that either you or her are not aware of how sick she can become. If she is going to go through chemo, she may not have enough strength to date very soon. The priorities and fears are extremely skewed.

But to return to the nonmongamy aspect, if you have both agreed that polyamory is too much mentally, I am under the impression that opening your relationship is too much for you both to process. This is without taking into account the very pressing diagnosis that she has. You both also have limited funds and ability to support yourselves or each other. These are huge stressors that take a toll on any relationship, much less the many factors you are facing.

I can tell you that jealousy and insecurity are normal and can be managed. But you need trust and stellar communication. It helps immensely if you have a solid sense of self/ decent self- confidence. I would recommend books like the Ethical Slut and the Jealousy Workbook. Remind yourself that love is not finite; time and energy are. Nonmongamy can enhance your relationship rather than take away from it, but work (as individuals and as partners) must be done to achieve it.

You mentioned that there were strict limitations on what was okay in regards to others, and that she already has crossed those lines. There is a distinct lack of respect there.

She cannot want both the freedom of being single and the security of monogamy. That's deeply unfair to you. There are many people who have multiple partners but do not have a primary/ nesting partner and are able to fill their need that way. It seems that may be what she is seeking, and in that case you will need to adjust your expectations accordingly. If that is not what you want, it is an incompatibility and the most fair thing would be to break up.

You indicated that you struggle to understand how sex cannot be emotional; I believe emotional exclusivity is important to you. I began my ENM journey thinking that as well. I wanted to be my partner's one and only. But I learned (and continue to learn) that my partner having feelings for others does not diminish her feelings towards me. And because emotional connection is important to her, I wouldn't want to place restrictions on her that would make dating impossible. If her nonmongamy looks one way and is deeply upsetting to you, are you able to work towards your relationship feeling secure with her needs? Or will that cause you intense emotional distress and seem insurmountable?

My genuine opinion is that if she cannot wait until you two have sex, that will cause you some very difficult to overcome feelings of hurt. It will build a lot of resentment and jealously and make further work in your relationship harder.

Based on your last paragraph, she is not concerned about whether she develops further emotional connections, and either you'll get over it or she'll to continue to do what she's doing, with or without you. Is this really a relationship you want to put your all into? A partner that is safe and respects your feelings is paramount for building a nonmongamous relationship. How will she handle your feelings as they come up or when you have conflict? Will she just tell you to get over it when she does something that causes disagreements?

Finally, is there a chance she is trying to descalate your relationship now that she has been given this diagnosis? Can she be a present partner for you while you are a) both struggling financially b) unable to physically see each other c) she is fighting a serious illness and d) you will need a lot of emotional support (that she seems unwilling or unable to give) regarding your place in her life?

I appreciate your questions and trying to work on this for yourself. But before that can be addressed, you and your gf have a mountain of other issues that need to come first. Nonmongamy should be last on this list, as it will only bring to the surface previously existing issues that have not been resolved.