Preface: I'm a Christian, so I'm going to try answer this question as a Christian. You've asked an honest question, however, and so I want to try to give you as honest of an answer as I possibly can.
Firstly, you're assumption is that her pursuit and attainment of something wonderful and good is somehow limited by your failure to pursue the same thing. In a nutshell, Jesus is more important to her than you. I think that's the sense that you probably feel, and I think you're absolutely right.
As an atheist, you may see that and think: that's ridiculous and calloused. But you have to see it from her perspective in order to understand it properly. She believes that being with God is a good thing, in fact, she would tell you, I'm sure, that it's the best thing that can possibly happen to her. Her heaven is to be with Jesus, who rescued her from the penalty of her sins.
So who, in this situation is more important to her? The person she spent tons of sleepless nights raising, whose diapers she changed and spit-up she cleaned up? (Not trying to shed a guilt complex here at all... I just happen to have a 3.5 month old at the moment, so these things are on my mind), or the one who came, lived according to the law, then died to pay the penalty for those who didn't, whose death has saved her soul for heaven? Which of these two has done more for her? Just like a new lover makes the loss of an old lover more bearable, and eventually insignificant, the love of Christ, in perfect union in heaven, will eventually make the loss of her son more bearable, and even insignificant in comparison to what she gains in Christ.
That's not to say that she doesn't wish that you would be with her as well. I'm sure she most certainly does. Paul himself speaks of this way when he talks of his Jewish brothers in Romans 9:1-3:
I am speaking the truth in Christ—I am not lying; my conscience bears
me witness in the Holy Spirit— that I have great sorrow and unceasing
anguish in my heart. For I could wish that I myself were accursed and
cut off from Christ for the sake of my brothers, my kinsmen according
to the flesh.
He has anguish for his brothers, but his anguish isn't going to cause him to abandon that which he knows is best for himself. While he can encourage those he loves to follow him in the direction that he is going, he can't force their hand. They're independent people. That doesn't mean that while he is here on earth, and while he can have some impact, that he won't hope for it. More than likely, your rejection of Christ is a very painful thing to her. Don't convert just to make her happy. It doesn't sound like you would do this, but if your only reason to convert is to make her happy, then you're not really a Christian. It's not about appeasing someone you love. Being a Christian is about valuing Christ, for his immense love on all of us who are fallen, messed up people, and for the sacrifice he made in our place. It's in this kind of conversion, that her value of Christ outweighs her value for you.
One last thought: Heaven is about Jesus. It's about Him. It's not about having a harp, sitting on a cloud, and partying all day. It's about Jesus. Put simply, God gives us what we want. If we find value in Christ and what he's done, and we wish to be with him, than God grants us heaven to be with Jesus. If we don't value him, then he'll also give us what we want: eternal separation from him. I'm certain that she would believe that the sense of loss you will feel having had not grasped Christ is nothing compared to the sense of loss that she will feel having had lost you. And again, this statement is not intended to belittle or minimize you, it's intended to magnify and glorify Christ. And Christ has stated himself, that those who seek Him will find Him. He will not turn away anybody who is looking to find him. But if you have no interest in Christ, then don't seek him.
For myself personally, I find His value immense, vast, and far above and beyond anything that I can find on earth. This is why I'm a Christian.