Use ～てしょうがない to say a feeling or physical sensation is so intense you can't do anything about it ("can't stand it," "unbearably"). It is the casual spoken cousin of ～てしかたがない / ～てたまらない, so it fits hallway grumbling and calls with colleagues but should be swapped for ～てしかたがありません or ～てなりません in reports and formal email. It attaches only to states you can't control — emotions, sensations, involuntary reactions — not to deliberate actions.