What a lovely letter of your feelings and concern. No, it is not incest. Of course it is okay for you to continue with something that is so life affirming. The relationship your mother has with her boyfriend is not at all an impediment to your relationship right now. The definition of incest varies by culture and society, but is typically considered to be intercourse between blood relatives. The definition can be used to refer to individuals living under the same roof, and those related by adoption or marriage. But since there are no bloodline connections, and your mother and boyfriend are only dating, you would be on safe ground. What isn’t clear is if they are living together or not. If they are, the struggle would be with the perception of others. If your parents don’t know about it the two of you should decide together if you want to tell them.
It is a wonderful thing to find someone who understands you at a level that makes you feel loved and safe. I would encourage you to follow your hearts.