Tough time potty training, help!

We are having a tough time potty training our son, he is 3.5 years old... we started a sticker chart today and if he gets 5 stickers he gets a reward. Goal is to get him to sit a bunch on the potty. He has had a couple accidents and now he screams and scared to actually go in the potty... he will sit on it tho no problem and understands what it’s used for and all. Any advice? We are having fun with the stick chart and will see if that will help. Maybe we have to take a break and start trying again in a week or 2? I’ve read you don’t want it to become negative at all

He held it for hours this morning and afraid he is going to get a UTI...