Don't ever feel guilty about anything, it doesn't help matters.

You both get to learn from this particular circumstance.
You're friend learns that if she is going to ask you for your honest opinion, she's going to get it. And eventually, she may figure out that its the hallmark of a good friend. She may just need to process your opinion, there's no need to apologize, especially if you really believe what you said.

You get to learn that when this friend asks you for your honest opinion, it may not be what she really wants. Use that information however you want. If she asks you for your opinion again and you want to tell her the truth but don't know how she'll take it, preface it with "it may not be what you want to hear, do you still want me to tell you what I think?"

Some tips:
-if someone doesn't ask for your opinion on something personal, they may just want to vent. If you want to weigh in, ask if they want your opinion. If they say no, respect that.
-remember that the friend asking your opinion is asking for help, so take into consideration your friend's personality to word your opinion in a way that they'll best hear it.
-You could stress that its only your opinion and that you trust that they can make the right the decisions for themselves.
-If you have personal experience, tell them about that instead of giving a straight opinion. Tell them how you felt during that experience and what you learned from it. (Even if they don't follow your advice, usually it gives them the awareness to learn from their own experience).
-refer them to someone else who has had a similar experience.

I tend to be a know-it-all and obviously, I like giving people advice. Sometimes people don't take it well, the tips I've given are not skills that I've mastered but when I use them, the message gets across more than with other methods, and I find that I'm able to be more helpful and less attached to the outcome.