play as one unit. its true u can't defend by yourself. especially the opponent are much better than yours.

in your case, i suggest u to keep communicating with your left wing to track back his area. when the situation like what u illustrate is happening, u might want to keep marking or close down on your area. thats when central defender or it might be safer to use defensive midfielder to keep pressure the opponent.

btw, i used to play as a left wing back at my school age. my coach always reminded me to track back the ball whenever the ball is on the left flank. another usage of left wing.