In other words, Romeo won't say anything to anyone, and is being eaten up from inside, as is the bud bit with an envious worm, eaten from the inside.
But he, his own affections' counsellor,
Is to himselfI will not say how true
But to himself so secret and so close,
So far from sounding and discovery,
As is the bud bit with an envious worm,
Ere he can spread his sweet leaves to the air,
Or dedicate his beauty to the sun:Literally translated: But he, counselor to his own thoughts and feelings, is to himselfI don't know how trustworthya counselor only to himself, [one who is] so very secretive and close-mouthed, that he is so far from any possibility of anyone delving into him and discovering the truth about him, that he is like the flower bud which has been invaded by a malicious worm before it can spread its leaves or bloom in the sunshine.