There are certain realities to modern professional life. Included among them is the truth that few of us have the mental bandwidth to handle more than one or two significant projects in our lives at any one time.
Thus it may not be a realistic option for you to try and take on a major writing project in your spare time while working at a time-consuming and draining job.
I guess you better forget about writing then--it's just not realistic. Yep, time to give up the dream.
Kidding!! Come on, readers of this blog know better than to give up that easily.
The solution is to take a sabbatical from your job.
Six months should be ideal--not only is it a standard sabbatical length, it's enough time to make an enormous dent in even the largest of writing projects.
And don't tell me your boss, or your spouse--or your bank account--won't let you take the time off. These aren't real reasons, they're excuses.
You are either a writer or you're not. Real writers don't tolerate excuse-making in themselves or in others. They shut up, set goals and write. Stop with the excuses and figure out a way to make it worth it for your boss and your colleagues to allow you to take time off.
And if you can't convince them, quit your job.
Finally, if you have a spouse or partner who won't "let you" take time off to write, perhaps you should seriously rethink that partnership. Should you tolerate a relationship with someone who isn't interested in helping you achieve your dreams?