M, it's strange to me that a lot of LBHs in particular really believe anything negative that their walk away says, maybe even more than anything positive. I fell into the same trap.
Here's why you can't believe what she says that's negative. Because she's trying to justify what she is doing. She can't reconcile in her head, "M is hot. We have amazing sex. But I still want the other guy." So she had to setup up that she's not attracted to you and that the SL with you is terrible so that what she is doing makes sense.
The other thing is the difference in men and women. For men, sex leads. If the SL is good then the relationship is good to a man. Women are the opposite. The relationship leads. If things are good in the relationship, then the SL is good to a woman. This is why LBH struggle because they are thinking, "if we can fix the SL, the relationship will follow". That will never work. So forget there SL and keep working on you, and be therefore by extension the relationship.
I can vouch that if you focus on yourself, the relationship will improve, and the SL will come along for the ride eventually.
I'll look for the thread where all that occured. It was in March of 2018, so I'm guessing it was probably in my first 2-4 threads.
Please do not ignore the advice on the birthday party.. Likely it's her trying to feel better about what she's doing. Don't fall for it. Likely you'll have another birthday next year. If things are better then she can throw you one then. If it's a milestone birthday, you can later tell her that there is nothing wrong with having a "41st birthday since I didn't give him a 40th".
Stay strong
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018