I know that Danzo had taken Shisui's right eye already and was after him, but why did he have to commit suicide? I know this question may sound simple, but I don't get why Shisui couldn't have lived on and help stop the coup the Uchihas were planning...
"Self-sacrifice… a nameless shinobi who protects peace from within its shadow… That is the mark of a true shinobi." -From Shisui to Itachi
Shisui gave his life and eyes in order to protect his home the village of Konoha, serving as an ANBU Black ops member. His Mangekyou Sharingan is one of the most powerful in all the Uchiha clan records that is why he thinks that his eyes can be used against Konohan whenever their clan initiate a coup d'etat.
Maybe Shisui can't live with the thought that he had to choose between the village and his clan.
Shisui was ambushed by Danzo, and ended up losing his right eye. This was a shock to him, because he served under Danzo as an ANBU Black Ops. He realized that there would soon be a coup d'etat from his own village, and he knew that his left eye would be a target from both Danzo and the Uchiha village.
Shisui did not want such a war between the Uchiha village and Konoha. He did not want internal destruction of his village, and he also did not want his village to perish. Thus understanding his eye as the most powerful tool, he knew everyone would go after him. So he believed it would be better to keep his left eye safe with Itachi, and commit suicide, so that no one would find his corpse, or go after him.