I remember reading a book back when I was younger, sometime in the 80s or mid 90s, I don't remember much about it except that it had something to do with when the Greek Gods and Goddesses lost too followers they started to lose their power and I believe some of them started to fade away, the more powerful Gods and Goddess where able to become human. I don't remember if they where born to human parents or if they just become very young and are adopted. If I remember right they don't remember they are Gods or Goddesses until later in life. I know its not a lot to go on, but being it was around 25-30 years ago I just can't remember more. I remember enjoying it and was hoping to find it to give to my niece to read, and read myself for nostalgia reasons.

Was it Gods Behaving Badly by Marie Phillips?

Amazon plot summary:

Being a Greek god is not all it once was. Yes, the twelve gods of Olympus are alive and well in the twenty-first century, but they are crammed together in a London townhouse -- and none too happy about it. And they've had to get day jobs: Artemis as a dog-walker, Apollo as a TV psychic, Aphrodite as a phone sex operator, Dionysus as a DJ.

Even more disturbingly, their powers are waning, and even turning mortals into trees -- a favorite pastime of Apollo's -- is sapping their vital reserves of strength.

Soon, what begins as a minor squabble between Aphrodite and Apollo escalates into an epic battle of wills. Two perplexed humans, Alice and Neil, who are caught in the crossfire, must fear not only for their own lives, but for the survival of humankind. Nothing less than a true act of heroism is needed -- but can these two decidedly ordinary people replicate the feats of the mythical heroes and save the world?

It looks interesting but that is not the one I am thinking of. One thing more I remember is the main character, a young woman in her late teens I believe or maybe very early 20s, goes on vacation I believe it is to Greece and its there that events start happening and she starts to remember bits a peaces. Also this was a book written for teens and it came out some time in the 90s or late 80s. I thought the author was Christopher Pike but I have been looking through his books and have granted found a bunch I fondly remember but so far not the one I am looking for.