Frustrated at 4 and a half

DS is 4.5 and still using diapers. He has asked several times to sit on the potty, stand in front of the potty, asking to pee. But he will never go. He then asks for his diaper. If he is about to poo, he will announce it, yet fights with great determination and persistence with the mere suggestion of going to the potty to do his business.

He has had 2 different kid potties and a comfy potty seat for the big potty and he just won't do it. I got him super soft underpants in his favorite color and character on them which he has worn twice but only for a moment. he doesn't like to wear them, possibly due to them being so thin and light in contrast to the diapers.

I'm becoming embarrassed to say my son is still in diapers. I'm constantly criticized about it. I'm being talked down to as though I'm a failure as a mother. But what am I supposed to do when I have done the bribes, tried the routines, tried different potties... when he just doesn't want to do it?

I have told him with each time I try to get him to the potty that if he does it, he will get a grand party and special treat ... but even THAT isn't enough. When he was 2, he actually did start to train and had three mishaps. One time the poo came out before he was able to get on the potty, the other two were pee on floor accidents from lack of proper aim.

These set backs were hard on him despite my praises for going in the potty. He then tried 6 months later .. unfortunately attempted to go himself without my knowledge and did not put the seat on the potty and his bum fell in and it scared him and he avoided it for 6 months.

Next time was another 6 months later and he gave up because he said he just couldn't do it. He then tried again and had to potty while at my father's. He asked to go in the bathroom and wanted to stand to pee, but when he couldn't, he wanted to sit even though I told him there was no baby seat on it. He wanted to try anyway and the experience scared him again and we were back to square one.

I'm at a complete loss. It's as though he's never gonna use the potty until his bum is big enough to sit on a full size commode and with each passing month, the special underwear are soon going to be too small and we have the largest size they offer. *sigh* IDK what to do.