Can anyone else suggest something interesting, please? What about using Seychelles jurisdiction? I read some threads recommending it.
Seychelles is not a suitable jurisdiction. Payment agent companies are almost never formed outside of EU because doing so has no benefit, and instead many drawbacks.
The limitation you're facing is that your company is not interesting enough to PSPs. This is usually a combination of volume being too small and the company being incorporated somewhere dodgy (Seychelles) or unsupported (USA). Based on the list of nearly 20 processors that have declined your business, it's probably both in this case, I'm afraid.
If you instead present an EU company, your chances of finding a PSP go up drastically. It won't be quite as easy as if you remove the US company from the mix and instead operate like a normal, law-abiding, tax-paying EU company, but more doors will be open.
Can anyone else suggest something interesting, please? What about using Seychelles jurisdiction? I read some threads recommending it.
Forget about Seychelels if you need a bank, payment processor or other financial service. It's almost impossible to find anything reliable or long time working!
Due to the difficulties to find a high risk credit card processor for my company based in the US (I'm a EU citizen with an online forex business), I was suggested to create an affiliate company of my US company in a different jurisdiction. Can this be an option? And if so, wich one could be recommended? I'm open to any new jurisdiction that can offer a merchant, account and a decent card processor.
UK, Cyprus and Malta could be possible options for you with good chances to get a merchant account. The problem will be FX business is deemed high risk and can be hard to place.
On the second note, you can speak with @Fred and @Gediminas both are well known to solve problems like yours.
Can anyone else suggest something interesting, please? What about using Seychelles jurisdiction? I read some threads recommending it.
Seychelles can be useful if you can find a bank or EMI to use. That's most often the difficult part for all of this. Unless you can use crypto only which almost no one can.
