Potty training 2.5 year old girl

I just had my son a month ago. My oldest is 2.5. I wanted to try and have her potty trained before he arrived but I didn’t think she was ready.

Today is the first day we’ve started really trying potty training. She has her own potty. I got stickers and little candies as rewards. She’s not interested. She usually loves stickers but when I told her she had to go on the potty to get one she suddenly didn’t care about them.

I have her in underwear and she’s soaked through 3 pairs this morning. She tells me she needs to go but not until AFTER she’s already peed her pants. She feels the wetness, then tries to sit on the potty. But then obviously she doesn’t need to go anymore.

When I’m putting her on the potty every 15 mins or so (sometimes have to stretch it longer when my 1 month old is ready to eat) it’s almost like she’s holding it until she stands back up and puts her underwear on, then she goes.

Any advice? I’m also exhausted from being up all night with my son and I’m finding myself getting really frustrated with her when I know it’s not her fault and she’s still learning. I’m completely overwhelmed lol