I know this is hard, but if you can, try to see things from her perspective. I am sure it was not sudden, she just could not express to you what she was going through. The suddeness was her decision to finally act upon her feelings. I am sure she questions her decisions, which is why she tells you she needs to think about everything. However, for your sake, you need to move on. You probably never will know what motivated her to make the decision that she did. I am sure she does care about you, she has just made some bad choices regarding the way she is acting. The right thing to do would be to talk to you first and resolve the relationship. Instead, she got involved with someone else first - probably because this made her decision easier for her - and, unfortunately, more painful for you.
I agree with CQ, you seem to be a nice, compassionate person and I am sure someone else will appreciate these qualities in you soon.