quit while one is ahead

as best one can

In the best possible way, given one's circumstances and abilities.

as far as one knows

To the best of one's knowledge, as far as someone knows or possible information to be gathered about the topic, subject, aspect, or situation.

back to square one

Located back at the start, as after a dead end or failure.

beat as one

To share the same feelings.

become one flesh

To join together in marriage; to develop a unifying bond as a result of marrying.

bite off more than one can chew

To try to do too much; to take on or attempt more than one is capable of doing.

bite the big one

To perform poorly; to fail.

bite the hand that feeds one

To cause harm to a benefactor (especially one that one is dependent on, thus with the idea that this behaviour is self-damaging).