Anyone not able to sleep unless they take Seroquel (Quetiapine)?

I've been taking Seroquel (Quetiapine) for almost 20 years. I have NO idea why the original doctor put me on it. I was new to psych meds and just did what they said. At the time it was depression and anxiety. The Seroquel helped me sleep, then after a while, I needed a higher dose, and on and on and on. I now take 500mgs at night. I've tried 3 times to come off Seroquel because I don't see the need for it. I'm now to a point that if I don't take the 500mgs at night, I never get sleepy and stay awake. One doctor took me off it and gave me Ambien. I took that every night for 3 nights and only managed to get a total of 3 hours over those 3 nights. It is like my body doesn't know know how to fall asleep without it. Unfortunately, I've been recently diagnosed with metastatic cancer (breast cancer that spread to my brain, no cure, just treat the symptoms if/when it returns) so they upped the Seroquel to try and help me fall asleep faster cause I've been a bit of a crying wreck over having Metastatic/Stage IV cancer. But I don't want to go up, I want to come off. Is it possible that I will be on this med for the rest of my life? My current med doc kind of sucks - so I'll try ANOTHER one. If I don't pass out from the Seroquel, I get these MAJOR cravings to binge eat. And it has always seemed to hinder my memory. So I really want off this stuff. But I drop 100mgs and I'm awake all night. Again, I have no idea why the original meds doctor put me on Seroquel, but I'm really over it after all these years. Sometimes I forget my meds at night and so I'm up all night until the penny drops and I go take them. Then it could be 2 hours before it kicks in or 6 hours. I just can't find the good in this med for me. I did go on to Effexor ER and once I missed one and it was like I was drunk! So I got off that quickly because many people said the withdrawal symptoms are hell for some people and didn't want to add something like that. Has anyone been on Seroquel, long term, high dose, and not been able to sleep unless they took it?

Depression
Anxiety
Agoraphobia

Seroquel
Buspar
Klonopin