Siebie/Sobie meaning?

Siebie 'Myself' I get, but upon looking for 'Sobie' in my Polskie Słownik it simply says 'See Siebie'.

Now the worksheet I have does this:

Zamknięty w sobie
And
Pewny siebie

So if I wanted to use: Ona jest pewna w sobie - That would be wrong?
If so, how about: Ona jest zamknięta siebie - Is that also wrong?

Just wanted to clear up the difference between Sobie/Siebie and when they can/cannot be used, thanks, J

enclosed in translates into Polish what??
you are usually sure of something not in something, right?

siebie and sobie are just different grammatical forms of the same meaning

you must have heard of noun cases

accusative - siebie
genetive - siebie too (od, do, obok (to, from, next to) require genetive (among others))
dative - sobie
instrumental - sobą ( z preposition (in the meaning of with - there is another meaning of out of/from) requires instrumental - z sobą - czuję się dobrze z sobą samym - I feel good with myself (about myself)

locative - sobie (w (in) and o (about) requires locative - so its: w sobie and o sobie -
czy widzisz w sobie jakieś cechy negatywne - can you see some negative traits in yourself
Kaśka nie lubi mówić o sobie. - Katie doesn't like to talk about herself.

I'm not far into study so those language cases are new to me, thanks for the detailed reply, I'll be careful to use it in the right context in the future, J