Relativity. I just can't grok it. I can't understand how time would flow differently depending of your speed. I don't get why C is a constant no matter the r

Relativity. I just can't grok it. I can't understand how time would flow differently depending of your speed. I don't get why C is a constant no matter the referential, for any other object the speed is relative to your referential. I just don't see how those 2 are compatible.