I know you're upset but this by talk a step back, it's not actually that bad. Everyone has had some feedback during their career that has been a little difficult to hear. In my own experience, the things that might have been upsetting where think I needed to hear and improved on given the feedback.
Firstly, there are very few managers that enjoy discussing sensitive topics or giving negative feedback. When it's done, it's primarily driven by a need for improvement and to help overall.
Regarding the shirts, take it on the chin. Some work uniforms are unflattering and badly cut and for anyone with a heat at all preventing gaping. I have a large chest so always size up in a short or blouse where I don't need to in a top or jumper. I also wear a cami or string top inside for modesty as I can't always prevent the gaping. This is very easily fixable, size up and a layer underneath and you are sorted and never need to dwell on it again.
Regarding the pace, improvement can easily come from a number of sources, 1. Additional training, 2. Watch & asking others particularly experienced people or those how just naturally excel at things, 3. Experience and 4. Confidence.
Would you consider going back in on your next shift and thanking the manager for bringing it to your attention, telling her that you want to solve the issues and you hope on a few weeks it will be a distant memory as you really like and want the job.
Require a larger size uniform or buy a new one if they aren't provide by the company - then it's fully fixed.
Then ask about the pace, as I'd their are team members she thinks are very quick, what do they do differently, what can you learn from them, what suggestions does she have etc.
This is very easily fixable and not something to get stressed about. Yes, being embarrassed and upset is a natural reaction but not one to dwell on. Try an be confident that you can change one quickly and you will also have support on the pace issue too. This is temporary and changeable.
You also need to go easy on yourself, saying you are good at nothing cannot possibly be true. You know that so stop saying it to yourself as it will only drag you down.