Abi, your friend is talking shit. The person you *really are* is versatile and has many interests. You don't get pinned down by fitting a certain mould, but instead you enjoy the many facets life has to offer. I think this is far more interesting than being a person who is always the same. If a guy has half a brain, he will see the benefits of you - that you can enjoy everything from Classical music to Rock. You can go from being girly to getting out and dirty and not caring about breaking a nail.
The one thing which is good to not vary enormously is your inner self. Being kind and thoughtful (but not a pushover) every day is the way to go. That's not to say that you can't have a bad mood and express it, just make sure to do it in a nice way. For example, some people have a bad day and cruelly take it out on their partner - but a nice person says "I had a really bad day today. I just need x and y. And if my mood is a little off, it's nothing you've done"
keep up being the interesting person that you are.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.