Hi L.,
this post could have been written by me! I completely empathize with your position. First, you must understand that there is nothing "wrong" with you. This sounds more like a case of balancing the more subtle aspects of your being. It sounds like you are doing well with exercising, good food, etc.
I recommend trying some complementary healing modalities. Acupuncture, massage, osteopathic care (or chiropractic care)...these can help get at some of the underlying issues of fatigue and stimulate your "life energy" or "chi." I have benefited tremendously from regular treatments, and in the process understand that my fatigue has more than physical causes.
Are you still nursing? If so, you may want to consider cutting back a bit. If you're not nursing, you could benefit from doing a cleanse--a practitioner like a certified herbalist can help you determine what kind would benefit you most: lymphatic, colonic, etc. and help you do it safely and gently.
Is your indoor air quality okay? Things like dust, smoke from petroleum-based candles, and mold can work against you--even if you aren't that "allergic" these operate on a subtle level to wear your defenses down.
Others are right to have you ramp up your Vitamin D consumption. Also, even if you are eating healthfully, you may find that you can work to avoid or limit even more the sugar, caffeine, white flour and heavy red meats--these are depleting by nature.
Don't underestimate the power of interrupted sleep. I still think you should aim for 8-10 hours of sleep a night because if you are getting interrupted, you need to make up those lost sleep cycles. Or, you could try cutting back to 8 hours, then napping with your son during the day. These strategies are something you could discuss with your doctor.
Above all, I recommend carving out some time for yourself if you can afford it or do a trade with someone. Take the time to read, or take a bath, go to a matinee movie, have tea or glass of red wine with a friend, take an interesting workshop, or go for a hike in nature. These and similar activities will rejuvenate you on different levels--trying for physical, mental, emotional, social, and spiritual wellness.
These may sound like a lot of work and they're not intended to all be done right away. Pick one thing at a time to tackle and add more in as you start to feel better. These are all things that have helped me to feel more balanced, less stressed, and therefore a better mom.
Best of luck and many blessings to you and your little one!