Me [35F] with my coworker 33 /F]-- how to respond when she pubicly puts me down?

We are both new this year. At first, she very aggressively spoke to me and everyone else about very personal aspects of her life, and she has had it hard, with a dead baby, infertility, a disabled kid. I tried to be understanding and express my sympathy and admiration about how busy she must be. I shared a bit of my own troubles, but in much less detail, but it quickly felt like she wanted it to be clear she's the winner, she's been through worse, which I absolutely believe, and the aggressive competativeness of it weirded me out, but I thought of her as friendly.

Soon, she was talking a lot with other women about her troubles, and I was a little relieved because it was overwhelming and I could not get a word in, even if it was a sympathetic one. At lunch, she is the same --there's one lunchroom with a big round table and she often dominates the conversation and especially wouldn't let me get a word in.

But she quickly turned against me. She started belittling me in private--- for example, I was once ten minutes late when I wasnt needed or missed and was actually helping a colleague with something work related--- "I'm sure if soandso-new-guy wanted advice, he wouldn't go to you!" or that I'm not doing a good job.

But now it has become public. She likes to belittle me in front of others or say very dismissively, "I dont know what youre talking about" and cuts me off when I exp!ain. She says, "you said..." and twists my words so im being quoted as saying something horrible. I had to tell the union rep after she publicly said that people hired under the policy I was hired under can be terrible, but they have to hire "them"-- there are only two people in that category, so everyone know who she was talking about. A woman came back who I had never met and she became friends with her and this new woman weirdly startedin on me, shutting me down whenever I speak at lunch, even when I just agree.

I tend to just say nothing when people do this. I'm just not good at witty comebacks and she is one of those aggressive speakers who just talks over people anyway.

I recieved an excellent review from the boss and the assignment I want, so I don't have a reason to be concerned that she thinks Im bad at my job, and we arent in any actual work competition, but knowing how competative she is, maybe my excellent review made it worse. And weirdly, I don't really want to be her friend or friends with the other women she seems to think she's formed the cool kids club with, so I'm not sure what she gets out of knocking me down other than feeling like at least she's not bottom of the totem pole. She is so aggressive about getting everyone's attention to talk about her problems that most of the times I talk to someone before or after work, people who arent even in her little "cool kids group", there she appears and the person I was talking to is just the person she needed to talk to.

I've started going for walks at lunch but I can't avoid her completely.

tl;dr: I feel like a very socially insecure and awkward woman at work has thrown me into the role of office Gary. What can I do?