Must be a difficult situation to be at after coming such a long way. Reading your situation, one thing for sure is, you are not dommed yet. You have it in you to study and excel, which is clearly indicated by how well you did in your Bachelors. So the first thing here that will help you is to recognize habits that are effecting your studying habits. Really, you must not be studying the right way. Now this right way may vary from people to people. So its really up to you to recognize/identify the best way for you to study. Please remember, things are not going to change over night and that you should really want and work for it to change.

From my personal experience I have realized that more you slack, more you will lose the capacity of absorbing knowledge. So if you really start studying the way that works best for you, things will only go uphill for you. Its all about making it habitual. I know getting low grades can be depressing. But deep down you know that the only person that can help you is U. So get started now. Start working on it instead of dwelling upon previous bad experiences. Make a schedule for yourself, change habits that negatively affect you (if you have any), and just have faith in yourself. You can get good grades!