〜てしかたがない／〜てしようがない

Meaning

Expresses that the degree of an emotion or state is so strong that nothing can be done about it. As in 「眠くてしかたがない」, it describes naturally becoming so and being unable to suppress it. 「〜てしようがない」 is a more casual way of saying it.

Examples

昨夜よく眠れず、今日は眠くてしかたがない I didn't sleep well last night, so today I'm unbearably sleepy.
彼女に会えるのが、楽しみでしようがない I'm looking forward to seeing her so much I can hardly stand it.