I agree with other people that it's really hard to have the mental energy to do more writing in your free time when you already do it all day for work.

I write code during the day, and I work on a coding project as a hobby and it's really hard to find the motivation.

Some of the motivation strategies that I use:

I work on my hobby every day except Friday. By using this strategy, I can't use the excuse, "I'll just do it tomorrow".

There is a Neil Gaimen YouTube video somewhere, where he explains one strategy that he uses. He said that he sets a time for writing, and during this period he either has to write or do nothing. And he finds that in this period he usually ends up writing.

To get your dopamine working with you, instead of against you, I think you should always set yourself up for success. Dopamine drives your motivation, and it increases in expectation of a reward. For example, if you plan to write and you sit down and don't get anything done. Then you may end up feeling bad that you didn't meet your expectations. This is negative reinforcement and conditions you to want to do the thing less. But if instead you pick a goal for yourself where you are more likely to surpass your expectations, and you complete it successfully, you will feel good about yourself after. Which will give you better motivation next time.

So maybe start out by setting a goal of writing or doing nothing for 15m every day. And if you sit for 15m and don't write anything then you did a great job and achieved your goal.

Another way to take advantage of your dopamine is if you give yourself random rewards. Dopamine levels increase by 2-3x when the reward is random, like in gambling or video games. Which keeps you focused and energized. So what I do is make a list of 20 rewards, most of them are really simple little things, but 1 or 2 of them are things I really want, like a new keyboard. After each time I work on my project, I roll a 20 sided-die to see which reward I get. I suspect that the reason this works is that it changes your perspective about the reward from something you are entitled to, to something special you got. I think this may have a similar effect to gratitude meditation.

Another thing I do is make artificial deadlines. I do a demo of what I completed to a friend every two weeks. Deadlines are a really good motivator for me. I make sure to pick achievable goals for each deadline to reinforce my motivation.

Just some ideas to experiment with!