Look for the Reason: you either want something or you want to avoid something.
I answer questions about Memory, so check out mnemonics for remembering important ideas. However, I understand the ‘fun’ element in your question and my answer is that you must be reading textbooks for a reason, so you must focus on the ‘want’ part of your reason– the desire for the outcome. This is the best and most positive way to motivate yourself.
There are two things that motivate me, desire to finish a project and interest and I think the latter is your problem. It is great that you already like to read, so you need to find something that interests you about the subject. If I am not naturally interested in a subject, then I will find something about it that intrigues me - a story, a new perspective, something amusing, odd, or unique.
Secondly, I assume that your desire is for good grades which will open the door to an exciting and fulfilling future. So, focus on that and plan short-term goals – with rewards - to give yourself an ongoing sense of achievement.