Run your profits cut your losses.

This comes from the world of investment where investors have discovered the wisdom of getting out of a bad situation early and sticking with the good. It can be applied to other life situations like dumping an unsuitable friend and working to keep the good ones. A good job is worth persevering with a bad one will never get you where you want to be. The difficulty sometimes is making a judgement as to what has potential and what does not. Obviously "flogging a dead horse" makes no sense but what if it is just sleeping? With horses you could ask a vet or a knowledgeable friend. In other life situations it can help to get good advice from a reliable source - don't let your ego get in the way.