Letting Go Of CRAPPY Friends

Why is it so hard to proactively decide to cut off a friendship?  I have a "friend" who is always critical and rude.  She doesn't contribute to the friendship, and most of what she says is condescending and snarky.  We hardly see each other in person.  I need to consciously decide to stop texting and break it off.  And I kinda feel like she wouldn't care, either.  So why do I feel so damn bad about it?