What do you mean by "uncontrollable"? Crying, in a rage, throwing up? Or jittery, antsy, etc?
Is this the first official team sport she's been on? Is the anxiety limited to just the softball practice or do other things (answering questions aloud in school, for example) also make her nervous?
Did you spend money for this softball experience? Did you buy a uniform or cleats or a glove? Did you pay to register her? If you did, I feel that needs to be taken into consideration. She's 11, not 2, so she has a basic comprehension of money. She shouldn't just be allowed to walk away from a commitment that cost the family money, even if you could comfortably afford it. She should consign her glove and cleats, and earn some money back to repay the registration costs by doing extra chores.
And make sure she knows that future begging and begging will not just be answered with a casual "sure!". If she begs to be on a gymnastics team or dance team, she'll have to put something into it, she'll have to save some money or earn some so that she's invested in this endeavor. She'll have to discuss the time requirements, the practices, the costs, and have a plan in place to deal with the stress.
If her anxiety is limited to softball, and it won't hurt the team if she quits (if she quits will they still have enough players?), then perhaps use this as a lesson learned, and let her mature a little more before committing to a team.
If her anxiety is all over the place, get her some therapy. Explain that it's like anything else - you need a coach or a teacher to learn something, whether it's sports or science or art. And a therapist can be like a coach, to help learn a skill that doesn't come easily, which in her case is dealing with anxiety.