I have great respect for you and anyone else who can do work a full time job while building a business of your own (same respect for those who earn college degrees while working full time).
The key lessons you'd get from a time management course:
1. Take care of yourself - sleep, diet, exercise - all the stuff they taught us 4^th^ grade Health.
2. Make lists, and prioritize everything on the basis of two things:
importance (how much real value it contributes to your life goals) and
urgency (how soon you have to do it before it becomes a bigger problem that requires more time). Block out large chunks of your available time to work exclusively on "important" tasks, with periodic short breaks to deal with any "urgent" things (which are usually important to someone else, but trivial to you in terms of their contribution to your goals).
3. Most "important" work can be planned in advanced, so protect your blocks of "important" time from interruptions. Be brutally selfish here, and remind anyone who tries to interrupt that you'll be available to deal with "urgent" tasks at 9:30 (or whenever).
4. When you take a break to handle "urgent" items, multi-task. Do two or three things simultaneously, and don't spend any more time on them than necessary. Finish them off fast.
5. Share your goals and dreams with others, and ask for their help if there is something they could do to that would allow you to be more productive. Be sure to thank them for each thing they do, and never stop telling them how important their support is to you.
In a way, working full time for another employer while working to build your own business is like having an affair (without all those troublesome Commandment-breaking implications). Some people may be able to do both for years, but most cannot. If your goal is to eventually quit your job and be in business for yourself full time, don't just imagine it -- set an expicit timeline and build a business plan that makes it happen. If you don't really believe that's feasible, scale back your revenue expectations for the business, at least for now, and enjoy the work itself. It's a cliche, but when you do what you love, the money really does follow.
Best wishes!