骑虎南下势成骑虎qí hǔ nán xià

ride a tiger and find it difficult to get off—have no way to back down; be in a situation from which it is difficult to extricate oneself;be caught in a dilemma (/a fix/an awkward situation); get into a position from which there is no retiring; get sb in the chancery;grab the bear by the tail (and can’t let go); have(/hold) a wolf by the ears; start sth that proves more than one can carry through without danger; up a gum tree
❍ 本月三日抛出的一百万公债，都成了～之势，…… (茅盾《子夜》300) …and the one million bonds we sold out on the third. It’s a case of“in for a penny,in for a pound，”…/然而为难的是现在两样都弄成～。(茅盾《子夜》346)The trouble is,we’re in both things right up to the neck,and we can’t easily give up any one of them.