1)miłość na zawsze = love forever (substantive+adverb)
2)wieczna miłość = eternal love
(substantive + adjective)
I don't know portugeese "amor eterno" it is "eternal love" or "love forever"?
practically the same sens end you can choose each variant, difference the same like in english.
Attention on orthography miłość (not miloszc!)
if you try google you can find both variants