Originally Posted by Chupacabra
I need advice. I don't want to break up with her...this is a last resort. I want to know how to handle this mentally and emotionally. How can I assert to myself that the amount of emotion she can invest in me can eventually equate to the massive amount of love she already feels for him?
Really, there is no way to assert such a thing because it may or may not even be true. No one can predict what their emotions will do in the future, no one can honestly promise to love you as much tomorrow as they do today. Oh sure, we all make such promises, and fully mean them at the time. But in reality, emotions change and we cannot always control them.
You need to realize that you did not "cave in" to her demands. While her communication leaves something to be desired, she's been basically telling you: "I'm going to start dating my ex. Get over it or leave." The fact that she would bring that into the relationship when she's not even sure if he's going to agree, and then to keep it on the table when he's on the fence about whether to agree... shows that she's got issues. Her issues, not yours, and there's nothing you can do to fix them.
This relationship seems dysfunctional. You seem to have an unrealistic sense of authority over her, in terms of whom she's allowed to date. She seems to be inconsiderate, e.g. implying that you'll never live up to the bar he's set with that love letter.
Of course she'll resent you if you try to end her other relationship. She's right to do so. You don't own her, you cannot control whom she has relationships with. I would say the same thing if you'd been married 20 years and only recently opened up; however, after only one month, that reality is a thousand fold.
You can only control you. She's made it plainly clear that she desires and intends to be in a relationship with that guy. Give up, immediately, on the possibility of him ever going anywhere, or of her being considerate of your feelings. Make a decision based on reality, not on what you'd like things to be. You are unhappy in this situation. You cannot change that situation. It may be possible to change the way you feel about them together, but you don't seem to be in the right place to do that. You're too attached to a specific outcome.
Yeah, breakups suck. But if she's treating you like this right now, after only a month, when she should still be in the honeymoon phase, why do you think it's going to improve with time?