How do I cope as a parent when my son has cancer for the second time?

My son is 26 years old and was diagnosed with colorectal cancer about 2 months ago. This is the second time he has had cancer. The first time he had a cancerous brain tumor. He hasn't shown much emotion since the initial diagnosis, but he did get very angry a couple of days ago. I think he is handling it better than I am though. He's a very strong young man. I, on the other hand, am having such a whirlwind of emotions, I don't know what I'm feeling when I'm feeling it. I go from coping with it to being angry, to questioning "why", to crying uncontrollably, to feeling defiant, to feeling helpless..... It's like I'm on this nightmarish roller coaster of emotions and I'm screaming to get off, but no one hears me. My chest is so tight, sometimes I feel like I have to fight to breathe. Sometimes I am nearly paralyzed by fear. Sometimes I feel like I'm trapped inside a huge black fog, it surrounds me and it's squeezing the air out of me and immense panic sets in. Some days I'll be just playing a game or watching TV, and tears will start rolling down my face out of nowhere. I feel so lost and alone. I know I'm not the one with cancer, and I feel selfish..... like I'm making it all about me....but I just need support to help me get through it so that I can help him get through it. I have absolutely no support system. There is no one who is offering to comfort me. There is no one to tell me it's going to be okay, even if it's so unsure. No one understands why I get so depressed that I can't even get out of bed sometimes. No one gets that I, myself, have many physical health issues which effect my mental state, and that when adding the fact that my first son, the first boy I truly ever loved, the one who stole my heart and still holds it to this day and beyond the ends of eternity and back, has cancer...AGAIN, it feels like more than I can handle. I feel like I will be crushed under the weight of it all. I need advice on methods others use to gain strength from someone who is, or has been, in my position. I need to talk to people who understand what I, as a caretaker, am going through. I apologize for this being so long, but I guess I just had a lot to say. May God bless and bring peace, hope, strength, and healing to everyone who reads this.