Is it still my fault if i rear end someone because they went to turn but didnt signal & stopped short?

You should be charged with following too closely. You have no legal defenses to get out of the ticket. You're guilty. As you said you messed up and now you face the consequences of that mess up. You were at fault. You should have been paying closer attention and not been so close you couldn't stop. Take this as a lesson learned and be more careful in the future.