Like many others I hold a few phrases as important in my life; They form a loose guide, an ethos as such, to the do's and do not's that I mostly adhere to in life.
One of them is:
If someone or something is negative don't go as often and don't stay as long, and if you can avoid going altogether, do so.
I apply this to people of course and have cut out many of them, those I feel have a negative influence on me, over the years.
I'm rarely selective either; Rather, I tend to determine if that negativity is of any value to me, which it never is of course, then cut that person out in whatever manner is appropriate. I do the same with events or activities that bring me those same negative vibes.
It's not always easy as sometimes that negative force comes from a person I've known for some time or a work colleague which can make it difficult to eliminate them although on those occasions I limit contact. It has a similar affect.
Cutting people loose like this has never failed to push me forward and on each occasion the only regret is not doing so sooner.
Taking control of the influences one has in life, making sure they are positive and uplifting rather than negative and draining can make a huge difference to attitude...And Attitude is everything.