If there is only one thing you would reliably do, it probably makes sense to establish a regular time to reflect on things. Not sure what is the optimal amount. One hour a week? Maybe make it a walk in nature/park, with pen and paper, so that you are not tempted to cut it short or start doing something else. Don’t use this time to force yourself to do anything. (Don’t do anything that would make you hate this ritual.) Just think, observe, and dream.
Establish good habits. (This sounds like a contradiction: “habits” vs “agenty”. The idea is that you are agenty about choosing and updating your habits, but you use the habits as the raw power to achieve the goals. For example, if the goal is writing a novel, the agenty thing is to establish the habits of regularly reading and writing, and reflecting on progress.) Regularity beats volume. If you can make yourself do one push-up consistently every morning, it is relatively simple to increase the amount later.
For inspiration, talk to other smart and agenty people. (May be difficult to find them. Perhaps I am prejudiced but I believe that “read/watch inspiring blogs/videos” is a wrong answer: famous bloggers will optimize for making you an addicted reader and potential customer, not for making you more agenty. Look for a relationship where you talk with someone as a friend, not where someone is trying to sell you things.) This gives you inspiration and peer pressure. Overcome your ego: people who are ahead of you are the best to learn from, but it may make you feel like a loser. (Maybe too much ahead is not good, because they would be too difficult to copy.)
Perhaps try meditation; one of the first achievements is the ability to turn off unwanted distractive thoughts when they come. Then, whenever you face an unpleasant task, precommit to work on it seriously for five minutes, and use the skill to turn off distracting thoughts during that time period. (You may be surprised to find out that it is actually quite easy to continue doing the task when the five minutes are over. Yes! But don’t push yourself. Precommit to the five minutes only; everything afterwards is optional.)
Read Don’t Shoot The Dog by Karen Pryor to get a “gears model” how conditioning works. (Sometimes things that work for some people don’t work for others, but for me this was the best book on the topic, and it explained so many important things, including some common mistakes people make. If there is one book people would ever choose to read based on my recommendation, it is this one.)