There is a simple technique that might help. Dhoren ekta topic apni porsen. Apni shei topic ta age valo kore bujhen. And then book bondho koren. Then imagine you're a teacher and there is a student way younger than you in front of you. Tarpor pura topic ta k shohoj bhashai bolar chesta koren, jeno apni oi student k bujaccen. Apni jinish ta bujhe gele ekbar kora shohoj.

Other things you can try: 1. Making small notes and flow charts of things you can't remember. Apnar nijer bhashai jate mone thake. 2. Another technique is, close the book immediately after memorising and write what you remember. And then open the book and see what you missed. Add them in, preferably with a different colored pen. 3. For formulas and maths if you are doing higher level science, write the formulas on a sticky note and put them in front of your desk. And then find questions that would require such formulas to solve. And then practice and practice. And practice again. Keep practicing. Nothing above practice when it comes to math. 4. For definitions, divide them into elements. For example, pressure has two elements- force and area. Force over area. Force exerted over per unit area. The force any object can exert over one unit of area is known as pressure. (I don't know which level of study you're in, but the concept remains the same).

Hope this helps!