Frustrated.....

We started potty-training Grant last June, when he was 2 1/2. He did really well in the beginning with just a few occassional accidents. Grant was accident free for months after the initial training period and then a few months ago the accidents started again and they are increasing in frequency. Jason and I are both at a loss. If we make Grant take breaks and sit on the toilet every 2 hours, he's good. But, we can't always remember to put Grant on the potty and then he will end up having an accident. Grant doesn't even tell us that he's had an accident, instead he will go to his room and change his pants or if we are out, he will just not say anything. I don't know what is causing this regression and I don't know what to do about it! Grant had two accidents at school last week and he's never had an accident at school before. I'm thinking that Grant just gets too busy playing and finds it more convenient to pee his pants instead of having to leave what he is doing and go to the potty. He only has accidents during the day - he's been perfect at night. Does anyone have any advice? I feel so bad because I loose my temper and yell at Grant for having accidents and he always puts his head down and stares at the ground:-(