A closer look
Could've is the short form of could have. It talks about something that was possible in the past, and it can also refer to an ability someone had, as in She could've won the race when she was younger.
I could've taken the earlier bus. The speaker did not, but it was possible. It could've been worse means the situation might have turned out worse.
You could've been killed! Someone says this after a near miss, like a car braking just in time. The danger was real, but it never happened. Could've here means a past possibility that did not come true.
I could've become the greatest player in the world. A player says this about a chance they had in the past. The ability existed, but it was never used.