Leah and Rosalie's whining.

Most of her hatred for what has happened is directed at the Cullens. They are the reason for the wolf gene being activated, and thus for the imprinting that comes along with it.
That's silly. She may have legitimate reasons to dislike the Cullens (they're her natural enemies, their son/brother is with the woman her friend loves, etc), but this isn't one of them. Like Rosalie, she too now has an awesome life, where she can be an awesome werewolf and stay young and healthy forever and not die. Like Rosalie, she should stop whining and be grateful about how much her life has improved.

Also, she and Sam were in a relationship. Which is completely different from Jacob, who was never in a relationship with Bella.
That is irrelevant. Bella's feelings for Jacob, or lack thereof, is a separate issue from his feelings for her. It makes no difference that she never loved him while Sam loved Leah at one point-he still loved her.

And that's why she's snippy with Jacob. He acts like his pain is worse than hers, when it isn't.
It was worse. Bella emotionally abused him and selfishly led him on for her own ends. Sam never did that to Leah. He treated her well, but imprinted on someone else. He didn't use Leah; it just didn't work out. This happens all the time-people break up and get divorced every day. It's not necessarily anybody's fault; it just happens. Bella, however, made a fool of Jake and as such he has every right to be upset about her abusive treatment of him.

And then, on top of that, she fears she is incapable of being a mother due to phasing.
The whole point of having kids is to replace yourself when you're dead. She's probably going to live forever, unless she gets murdered by a vampire. She doesn't need kids because her DNA will still be here, whether she breeds or not.

She had a fairy tale human life. She was blissfully happy.
And she ended up with an even better life, thanks to Carlisle.

And then the man she loved rapes her and lets his friends take turns with her, leaving her dying in the street.
But that's not what she's complaining about. She's complaining about the vampire stuff. A lot of people get raped, but they don't whine about their circumstances improving, as hers did. Also, because she was a vampire, she got to kill the rapists. How many human rape victims get to do that?

Then she was burnt to a crisp in the turning process, without understanding what was going on.
That was temporary, and quite short compared to eternity.

But most women have a deep desire to be a mother, and some even go through all kinds of horrible procedures to have a child.
Nature does this in order to protect the species from extinction-the organism will need kids to replace them when they're dead. Otherwise, when we die, the species dies too. Since Rosalie and Leah will probably live forever, this does not apply to them.

Some even kill pregnant women and steal their babies to be a mother.
Only those who are seriously deranged, probably to the point of insanity. It serves no evolutionary purpose to steal and raise someone else's kids-remember, these kids are your kids competition. If you help them survive, you are putting your own kids at a disadvantage because the more competition they have, the harder it will be for them to find food and mates. No normal, sane female would do that.

And for a lot them, it isn't about having someone to carry on after they're gone.
That's the whole point of having kids. Nature doesn't care about the individual; it cares about the species as a whole. It's not about the individual's self indulgence; it's about protecting the species from extinction.

For them, being a mother transcends being young and beautiful forever. Like Rosalie, they'd give up being a vampire to have a human life as a wife and mother.
Now that's just silly. Rose was given an amazing opportunity that most people would never have, and she's butthurt because she can't play house with a cute kid? Come on. Human life is temporary-which means that by extension, motherhood is too.

That isn't selfish.
She's selfish because she wants to deny this awesome lifestyle to everyone else because she doesn't like it. She is denying other people's autonomy and imposing her own preferences onto everyone else instead of respecting their choices. All she does is complain about Bella wanting to become a vampire, and she hates her because of that. Granted, a lot of people dislike Bella and justifiably so, but this is different-Rosalie's reasons for disliking Bella are stupid.

Choosing to be a sterile immortal over being a human with a family is selfish.
How is not wanting to die selfish? All organisms have a survival drive-nature made sure of this because we'd go extinct if we didn't. We are all biologically programmed to want to live as long as possible. That's why the suicide rate is so low, probably lower than statistics indicate because of tabulated suicides that are actually accidents, or a host acting under the influence of a parasite. By your logic, every single organism on the face of the earth is selfish because we haven't killed ourselves yet.

And how is it selfish for a woman to not have children? What a sexist comment.

Have you ever, in your lifetime, been in love with someone? And if you have been in love, have you ever lost that someone to another person? Do you have any idea how excruciatingly painful that is?
Sure. We've all been dumped; it happens. The difference is, though, that love is ultimately a choice. Leah can stop loving Sam if she really tries hard enough. Sam is physically unable to stop loving Emily because of the imprint, so she really has no right to be angry at him or Emily. What are they supposed to do, let Sam die and/or be miserable forever so Leah won't be offended? It's actually pretty selfish and self entitled of her.

thought I loved
There's no such thing as "thinking" you love someone-you either do or don't. That you don't love him anymore does not change the fact that you did at the time.

I had to sit back and watch while the one man I loved (or, at least, thought I loved) ended up with someone else.
He chose to love this other woman; no such thing applies to Sam and Emily. Imprinting is involuntary. How can Leah be mad at them for something beyond their control?

And I know, all too well, that it isn't something you just, "get over." It takes time.
Leah's had at least a year (maybe longer-we don't know exactly when Sam phased because Bella didn't know about the werewolf stuff until New Moon) to get over it. And it's really not even that much about her loving Sam, because she told Jake that she would love to imprint or be imprinted on so she could move on-demonstrating that she is in fact ready to move on. So why doesn't she date someone instead of sitting around and whining about not imprinting?

Do you know what it's like to live with the knowledge that you will never be able to conceive a child? I don't, thankfully. But you know what? I have enough common sense to know that it's not something you just, "get over."
It's not that bad. If she thinks kids are that cute, she can always adopt one and pretend it's hers.

Also, you should keep in mind that Rosalie didn't choose to become a vampire. She didn't sign up for an immortal life. That choice was made for her. And it's because of that, that her chance to grow old with someone and become a mother - something that she wants more than anything - has been stolen from her.
Yes, Carlisle "stole" it from her but he gave her something better. "Oh, poor me, I get to not die and be young and healthy forever! " GMAB. Her life is awesome.