I wanted to add a bit of backstory as it may help to understand what I'm facing, although I don't understand why or how.
I'm 37 now and when I was 26, I had severe bulemia. I got all the help I needed back then and I even had to have my gallbladder removed due to extreme weight loss and gallstones had a field day. I was put on Citalopram (Celexa) and it worked wonders. 
2 years later I stopped taking them and I felt great. Until my Dad was diagnosed with terminal cancer, out of the blue. I flirted with the idea of going back on my meds as I knew they would help me through what actually turned out to be the worst 15 months of my life. My mum also broke her back and they were both bedridden. Again, I felt the  work within 2 days and I could function.

About 3 weeks ago I work up with the most crippling anxiety. I've never felt anything like it. My stomach was in absolute knots and I can't only describe it as feeling so desperate that I wanted to run and run away from myself. I got straight on the phone to my GP who started me back on Citalopram (Celexa) and I was hopful due to how they have worked so well the past. Not this time. It's been 3 weeks of all I can describe as sheer hell. I getting 2 hours sleep if I'm lucky and the minute my eyes open, the anxiety hits me like a ton of bricks. It's overwhelming and the only brief release I get is to cry and sob for a while. 
My doctor has called me twice to ask how I'm getting on and I tell him nothings changed and he just says he will call again in a week and that the tablets may get worse before they get better. That's never happened before with these thoght, but I've also never taken them for crippling anxiety.
I'm in the UK an so they won't prescribe certian meds frequently here like Diazepam.
Any advice on how I can see a light a the end of this horrendous tunnel?  All advice welcome. Thank you for reading this.