Carve the peg by looking at the hole

A kitchen knife cannot carve its own handle

You had better be a round peg in a square hole than a round peg in a round hole. The latter is in for life, while the first is only an indeterminate sentence.

So I'm a square! Well, at least I'm not a square peg in a round hole.

Digging a hole for yourself

Dig yourself into a hole