How dangerous are opiate withdrawls?

So im a 21 year old male, and ive been abusing prescription painkillers for about 2 and a half years. Pretty much all of them vicodin, oxy, morphine, methadone ect. you name it ive abused it. And now im really considering quitting because every morning when i dont have any pills i experience severe withdrawls. ill feel sick to my stomach, aches. but what scares me is that whenever i stand up ill get really ligth headed and feel like im going to pass out. and at the same time my heart will be pounding but will feel like i have low blood pressure or something. I know anxiety is a symptom of withdrawl, and on top of that i already have an anxiety problem. so im not sure if what im feeling is just anxiety or something more serious. I know that was a lot to read but i guess what im asking is is this serious? has anyone else experienced this? Any real answers appreciated.