Ah - well to be honest, for me I find flat rides a lot scarier than coasters. I flat out refuse to do Detonator ever again and I’m not a massive fan of Rush and Vortex either.
I know it’s not easy but you might have to just grit your teeth and go for it. When I took my niece on her first upside down coaster (Pinfari ?), she freaked out going up the lift hill so I just stayed calm, got her to sing and scream and it worked.
Stealth used to scare the ? out of me until my 20ish ride on it and the only thing that helped was screaming/swearing on the way up - not very ladylike but it worked.
Inferno is a great one to start with - it’s the first one I did at Thorpe and the same with my niece. I remember being scared of the vertical loop going up the lift hill on my first go thinking “sh£&& that’s high” but you really don’t notice it - as in it feels a lot different to how you would expect it to. If you are concerned about motion sickness I can highly recommend cinarazine (Stugeron) - without that stuff I would be so unwell on rides. Obviously you are anxious but if you can get hold of some, Rescue Remedy is really good..
I’ve probably repeated myself from older posts but I’m too lazy to go back and check ? so apologies. Finally go with someone who understands your fear and will reassure you but not bully you or pressurise you to do something if you’re not ready.
If someone had told me 6 years ago that I’d ride all the coasters at Thorpe, Towers and BPB (plus Efteling and Phantasialand) I would have laughed in their face but I’m glad I took the plunge - it’s just that first step to overcome then I’m sure you’ll be hooked