You should say "Fui feliz (por muchos años)" or: Yo era feliz hasta que...

In Spanish, you cannot "*estar felizmente" (although you can "estar felizmente X", where X = "casada/soltera/"...etc.), and you cannot "*ser felizmente X" (X= anything) at all: "*Yo estoy/estaba felizmente", "*Yo soy felizmente guapa/soltera/norteamericana" are all ungrammatical. If you want to say that at a certain extended period of your past life you were happy, you have to say "[description of period + yo ERA feliz", e.g., "Cuando vivía en Nueva York", [yo] era feliz". If you want to specify an event that marks the boundary of that state of happiness, you must say "yo FUÍ feliz + HASTA QUE [+ whatever changed that, e.g., "perdí mi trabajo"/"vine a Berlin"] OR "Yo fuí feliz MIENTRAS + [state of affairs that made you happy, e.g., "viví en Nueva York"]. However, you CAN use BOTH "era" and "estaba" followed by an adjective ("Era feliz"/"Estaba feliz"), but with a subtle difference in meaning: If yours was a general state of happiness, choose "era"; if you are referring to some particular circumstance that made you happy (in some department of your life), but do not want to say that you were generally happy, then choose "estaba". E.g.: "Cuando mi jefe me consultaba para todo, yo ESTABA feliz (in what concerned your working life). That statement remains compatible with your being less than happy in all other departments of your life (no friends, no sex life, bad health, loneliness, boredom, ...whatever). If you say "Cuando vivía en Nueva York/Mientras viví en Nueva York yo ERA feliz", that is INcompatible with any unhappiness at all, in any aspect of your life. If, at that time, you were lonely, had no friends, were poor as a rat,....whatever, that still did NOT make you unhappy. Finally, if you say "Mientras viví en Nueva York FUÍ feliz", the choice of "fuí" automatically entails that your happiness ended when you left New York.

I think the correct word is "estaba".