You should be able to work with OPT for one year, this has to be arranged through your school. After that, there are no visas for new nurses, even with BSNs. This means noone can sponsor you, even if they wanted to. There is a very long wait - you will need to look to other countries.
May sure you apply for OPT before you graduate, I think at least 4-6 months before you complete your training. As mentioned even then once you complete OPT unless you stay as a student you are looking at another country or returning to your home country. Retrogression is not something new and your international student advisor should have told you about the difficulties staying once completed training
You can get OPT again as long as the course you do is higher level than the one you used previously. i.e if you did ADN and use OPT and then go on to do BSN then you can use OPT again as BSN is a higher level than ADN