J.,
Why not listen to your instincts, which are clearly telling you not to wean yet? Your "guilty" feeling and the difficulties you are experiencing may be because you, and your son, are not quite ready to stop the breastfeeding relationship. I know how it feels to want to wean, I've been there, but looking back, I wish I had listened to my own body and the signals I was getting from my daughter, and continued a while longer. Later, with my son, I nursed for two happy years. It wasn't always easy, but the RELATIONSHIP you develop through breastfeeding is so precious. Also, the longer you nurse, at least 18 months, the more protection you have against breast cancer. And your children will have a higher IQ, which has been proven now in studies. They will be protected from ear infections, allergies, intestinal problems, a whole host of things -- all because you had the commitment to give them the very best.
This doesn't mean you can't continue to feed solids and let your child drink from a sippy cup -- it just means that at those precious times, like right before bedtime, you and your son continue that bond, and you continue to give him the only real "vaccination" against disease.
That's my two cents, but I don't know your reasons for wanting to wean. I just personally believe very strongly that when a woman fights her own instincts, she is doing harm to herself. Give it some thought.
S.