Do you need a glass of wine? It sounds like it. I had to go read you prior to post to properly respond. First things first, breathe and calm down.
You're in a triad with J and R. R and J don't trust each other completely, and J wants to transition from romantic partners to metamour. Right now, R is unaware of J's plans to break off the relationship. He's conveyed it to you, but he's leaving 1/3 of the triad out and in the cold. That 1/3 is the most important piece because he's under the impression that he's in a relationship with you both. Once he ends it with R, it will go from triad to a V, where you would be the hinge, or the common love interest. I hope I have that all together.
R - Boyfriend of 5 years. You're not sure that you're in love with him.
J - Boyfriend of 7 months. Marriage is looking like a possibility. NRE? Maybe, or he could be the man you marry.
You see marriage and children in your future. OK. That's possible. With your situation now? Iffy. You stated in the original post that R and J only agreed to this triad to make you happy. Their relationship started wrong. They should have just been friends/metamours. That could have been alleviated and hearts wouldn't have been broken if they knew they had no sincere romantic interest in each other.
What does R want that you can't give him? You stated that what he wants isn't something you can give him, and that if your relationship ends with him he'd feel like a "loser." Why does he feel that way? Losing you to another man? Losing you as a whole? Clarification please.
Here are the issues at hand:
You are with R and J. You know you love them both, but you can't say that you're IN love with R. Are you in love with J, and can you see yourself being married to him and having children with him? Is marriage something that J wants with you? Is marriage something that R wants, too, and you know that you can't give him that? You could have a triad and be married. That's what my situation was. We've transitioned back to a V, and I'm the hinge again.
R and J don't trust each other or even love each other from the sounds of it, so their relationship needs to end. They need to talk and handle that. I would stay out of it. I know you probably want to protect R's feelings, but J needs to be a man and step up and be honest and stop playing with R's feelings and heart. As soon as he realized it wasn't going to work, he should have been talking to him and stop wasting time prolonging the inevitable.
You need to figure out what it is that you want and what type of dynamic. You want to get married. You want to have children. Awesome. Do you plan on continuing to be poly after marriage? Do you have hopes for another triad, or would you prefer a V? Are you hoping that you can marry J and possibly maintain R as a boyfriend while giving him the option to seek someone else like a wife and have a family of his own? Granted everybody has to be up to speed and aware of what's going on.
Is your relationship with R worth saving and putting forth the effort to rekindle that flame, or can you say for sure that it's just not worth continuing and you'd rather set him free?
I can respond to this a few different ways, but I need more information.