Your mum is trying to emotionally blackmail you into not transitioning. That's because as far as she's concerned, she gave birth to a daughter and that's what she expects you to be. Some parents are like that, sadly. She's desperately searching for anything that might support her position. There are several 'sex change regret' websites out there, but almost all of the stories on them are either spurious, or cases where people were not properly diagnosed before transitioning. Those cases are extremely rare, but they have really happened... which is why we have so many safeguards in place today.
Most of the cases of post-surgical suicide are not due to regretting surgery, or regretting transitioning... they're because society continues to treat trans people (particularly trans women) like we're sub-human, and some people's results don't enable them to blend into society in such a way that other people treat them like ordinary human beings again. Some people can't cope with society's continued rejection, hence those suicides. I'd recommend that for every link she sends you of 'trans regret' stories, you send her 5 links of stories of people whose lives have improved beyond measure by transitioning. Bombard her with positive stories every time she sends you a negative one. Eventually she'll give up.
I can only speak for myself, but I can tell you that transitioning has been the greatest gift I've ever given myself, and I have zero regrets about top surgery. If you feel anything like I did about my chesticles (abject horror & disgust comes to mind), then you'll probably find top surgery incredibly life-affirming and liberating. Ask your brother what he'd do if he suddenly developed a set of DDs. When he stops s->-bleeped-<-ing, point out how it would feel to walk down the street: how people would treat him; how other guys would make lewd suggestions to him - just for walking down the street with a massive rack on his chest. Wouldn't he feel horrendous about it? Wouldn't he want to get rid of those things so he could just feel 'normal' and so his clothes could hang right? Of course he would - so he's talking out of his backside when he says it wouldn't fix anything.
I don't know where you are in the world but as Dena said, if you're considered an adult in your country - or are above the age of medical consent (which is 16 here in the UK) - then your mother cannot stop you from having any medical treatment you desire. But of course, your family dynamic may complicate things. It sounds like your mother wields a lot of power in the home, which is why she's piling on the pressure to keep things just the way she likes them. That's an incredibly selfish position by the way: what could be more selfish than asking someone else to live a life of misery just so you can be comfortable in how you view them?
As you've already scheduled your top surgery, I presume you've already seen a gender therapist and have a diagnosis of gender dysphoria. Have you considered taking your mother to an appointment with the therapist so they can start explaining things to her? Have you considered asking your mother to speak to other trans people, either in a local support group or online (such as the SO forum here) so she can start to understand? It would help if she could get some positive stories, because she needs to start accepting her reality. She gave birth to a trans son, not a daughter... and that fact will not go away just because she'd rather that were not the case. The sooner she comes to terms with this fact, the easier it'll be for everyone involved, including her.
The obstacles she's trying to put in your way are not effective, but her behaviour will cause a lot of unnecessary pain & trauma for your family. Especially for you.
