Hi, and welcome to the forums here.

Someone a bit more familiar with Portugal will have to explain the visa situation, but generally it isn't easy for US citizens to just pick up and move to the EU. If you're going to work, you have to show that you aren't taking a job away from a Portuguese citizen, and if you aren't going to work, you have to prove you can manage without working (and provide your own health insurance).

Don't worry about losing your US citizenship. It's getting more and more difficult to lose it, or even to give it up voluntarily. You will have to continue to file US income taxes, though there are tax treaties to try to avoid double taxation of your income. You may want to try and find the website of the US Consulate/Embassy in Portugal. They usually publish quite a bit of information for Americans living in the country.
Cheers,
Bev