"So silly" means that you think you were very, very silly. "Silly enough" means that you think you were silly, but only enough to believe him. Think of it like a glass of water where the silliness is water. If you say you were "so silly", it's like the glass is overflowing. If you say you were "silly enough", it's like the glass is filled to the top, but not any more.

I think the first one implies a greater degree of silliness on your part.

I think the both are right, and have the same meaning becuse we have structure: be adj to do sth ( be so, too adj to do sth) => I was so silly to believe him be adj enough to do sth => I was silly enough to believe him. (I think, I don't use this sentence in this case, because be adj enough to do something means have a condition or ability to do sth . ex: he is tall enough to play basketball. hope it help you. have a nice day.

They are the same but the first has a bit of bad grammar, The second one is good.

Thanks! That really helps！