How to concentrate. And mentally?

First you need to organise your syllabus and a study plan (the materials and book you need) that you could cover in 4 months. You have to plan for a minimum time that you could spend per day. At the beginning start with easy subjects or some material that you are strong at. Try to stick with your plan for at least 2 weeks. You have take this period as a trial period so that you would know how much time you could really spend and incorporate this experience with further plans. If you could not stick with your plan don't criticize yourself. Try to learn where you make mistakes. Most people try too hard and give up easily. So to avoid that mistake you need to start with easy subjects. This is not to underestimate your capacity but it's a time period to get back to your form. It's like a warm-up game. Some success with this can give you more confidence to go further.

remember the first 2 weeks are trail period. you start to study for 1hr and if you complete then stretch some more time.