Sugarbear;
It's ok. You're entitled to feel. Sometimes a "cheer up" or "hang in there" can never suffice nor can anyone ever understand what and how you feel in your situation.
I questioned why my father died of cancer when I was 18, leaving me to care for my grandmother and deal with his new wife at the time, an ex-wife who hated me and leaving behind my half-brother and sister who were 7 and 5 at the time. Sometimes I would go to his office, and his clients would come by the office and ask me to help them with their problems from divorces up to murder cases (again, I was 18). My step-monster at the time robbed us blind, and I hardly ever saw my siblings until later.
I always wanted to go back and ask my dad what he thought about the situation, beacuse I was with him a lot and in the end. One of his friends told me my dad said to him, "I wonder why its taking so long.." He died within 4 months of his diagnosis at the age of 37.
None of it made sense. To be honest, it still doesn't.
I don't share this to seek consolation for myself but to show you that it's OK to have your down days. We each have our ordeals. Your son has to live with his situation everyday. He can't escape from his body. But he has you, and that's worth a lot. And you still have him...