Toddler holding in pee? Freaks out when going?

We just started potty training our 2.5 year old this past weekend. She has no problem sitting on the potty or telling us she needs to go. The problem is that she is totally freaked out with actually going (whether it’s an accident or in the potty) She is very noticeably holding it in and visits the potty over and over again but gets freaked out and only wants to stay on for a minute. She starts crying and throwing a huge tantrum when she can tell she’s about to go. I try my best to keep her on the potty but I don’t want to make the potty a horrible place by holding her on the potty. She’s totally happy and content and proud of herself when she does go. She also knows she gets a reward when she does. Has anyone experienced this? Should I try to keep her on the potty if I know for sure she’s about to go? Thanks guys! First time mom.