Oh gosh! What an experience you've had!
Nickel allergy is quite a big concern for us dentists. I'm glad that you now know that it was the nickel.
When it comes to braces for adults, there are three main options:
Fixed options, these stay on your teeth, you can't remove them as you wish:
1. Metal ones - cheapest but as you have said, they are very visible.
2. Ceramic/tooth coloured - more expensive than metal ones but more subtle. Ceramic also stains so you have to avoid curry etc. I have ceramic ones.
Removeable:
3. Invisalign/ clear aligners - a bit like retainers, you can remove them when you want but they are the most expensive. The range and type of movement is a bit more restricted with Invisalign compared to the metal or ceramic ones. However the technology is constantly developing and they do work really well with some cases. You'd have to speak with your orthodontist whether Invisalign is good for you, I can't say without seeing your teeth and plus, I'm not an orthodontist.
I did not want the metal ones so for me the choice was between ceramic and Invisalign. My pain threshold is -1000 (I stub my toe and lie in bed for 3 hours). Invisalign requires very good compliance from the patient and confidence that you will wear your aligners for 22+hours a day. For me, I knew that as soon as it started becoming sore, I'd just remove the aligners so I didn't trust myself with Invisalign. So I needed something fixed and ceramic was the answer for me.
If you like, I can expand more on the type of braces and I hope this helps!