I sympathize with your predicament. Many people have such issues to greater or lesser degrees. Discipline and motivation are somewhat different skills, though.
I would suggest to you that you not focus on long term discipline. This is primarily a matter of developing the habit of certain activities. Motivation is enough to begin with. Not merely for the practices we have discussed, but for the mastery of any skill... including your writing. As someone who also is daunted by the sheer time investment needed to churn out fiction, I relate to you quite well. And, it is never too late to write... it is one thing that you can never get too old to engage in.
A slight change in attitude and focus is all that is really required. If you recognize the benefits of a certain activity, there is motivation enough in this. Don't project into the future or lament the amount of work that lies in front of you, just simply start the activity and plug away one step at a time... try and stay in the moment. When you reach your attention span limit or become exhausted, ask yourself if you can tough it out for another minute, and do this 5 or 6 more times before taking your rest. In this way, you will increase the amount of time before such fatigue sets in next time. It won't be long before you can put a good amount of time into the activity in question.
This is similar to hiking up a mountain. If you only dwell on how far you have to go, you will get discouraged. Merely start to walk, and focus on the sensation of each individual step. Rest when you require it, but push yourself a little bit each time you set out and soon the whole thing will become easy, as you develop the requisite stamina and conditioning.
Your main enemy in mental disciplines is the wily ego voice which whispers to you all manner of bullshit trying to discourage you from any activity that it feels might threaten it. Meditation and yogic practice are damn near the top of that list.
I am not going to discourage you from using entheogens to find your answers. By all means not. I will only wind this up by saying to you that entheogens only rarely give you the entire cure for what ails you. They are awesome at showing you where and what to work on. And, combined with mind work and a good deal of focused intention, you can make lasting changes... but psychs are often like the coming attractions for a film. They have glitz and flash, and might give you a decent idea of if the film is worth seeing... but they don't give you the whole story. You can even be left more confused about what the film might be about. It can even be that really awesome previews mask a rather tepid film, or (usually) the other way around.
Doing the inner work is seeing the film. It is slower and less intense and packed with action, but at the end of it... you know the whole thing.
All the best...
HF