Why are bi polar people so nasty?

i hung out with a girl for 3 years and really never totally understood the whole bi polar depression 0disorder .recently outt the blue i received a nasty letter from her out of the blue basically taking me apart and critising me in the note ....keep in mind she was at my house 2 days previously in ok form or so i thought .the thing that makes me so mad and kicking myself is ive divulged alot of information about my life personal things and basically put trust in her ...like an idiot !the last few days have been hard as u nearly start believing the abuse she wrote in the letter .my understanding of bi polar is that its a sick nasty condition that leaves the non bi plar person absolutely baffled .she lives alone and is single and doesnt like taking her medication ..so reall there is no supervision on whether she takes it or not .the problem is she lives right next door to me and now im creeped out by her ....how did i hang around that kinda **** for 3 years and let things slide for so long????i dont care what anyone has condition wise ..i feel she is well aware that she hurts people and even said in the letter she knew id be angry with receiving it ....sick of people making allowances for people with bi polar that can get away with saying what they like to people but can seem to come across "together"can make articulate conversation etc ...i know she has bitcjed about me to a neighbour and im getting to the point that i wanna slapp her face for this uncalled for **** .ive completely ignored her and have not responded to the letter and remain to do so .is ignoring her completeley the best bet for me ????i cant take much more **** ....