I understand. You can try to sit for 15 minutes and take a one minute break. Break attention intentionally and try to then gradually increase from initial 15 minutes. Also if you force yourself to understand in less time, it will appear tedious. Forcing yourself to learn from one source will also make you feel frustrated. Focus on learning the topic and focus on getting the output. Try smaller sessions, explore topics from different sources and try taking notes and revising what you go through. If you still cannot, ask anyone if they can explain the topic to you, not necessarily everyone can make you understand but maybe some can.