Here's what I think the mv means:
So San E is dead.
That scene where he gets hit by a car in the beginning, it means he died. In fact, you can see the guy who hit him looking over his body in the background. So this was basically after the breakup. The scenes where him and his ex are eating is actually his ghost sitting in place of her actual current boyfriend. She leaves because she is either heartbroken about San Es tragic death or she simply just misses him and keeps on thinking about him instead of her current boyfriend. Then she comes back, cleans herself up and let's go of San E and moves on to accept her boyfriend. That's why San E fades away in the end and it says " I think I can let go do you now"
But idk that's what I deduced...