A paragraph in my book that is nagging at me. HELP

When a friend tells you a secret, something which they could trust no one else with, you are naturally expected to keep it, after all promises are promises. However sometimes your trustworthiness can be over estimated and your head can fail your heart. I remember when I failed my friend and did not keep my promise of secrecy. I had expected rage, hate and bitterness but her reaction shocked me to my very soul. Her eyes filled with disappointment, shock and most of all regret. I had laughed and scorned with the crowd but I cried out bitterly inside. Questions flooded my mind: Why? What was I thinking? How? Regret shall always be the most brutal of tormentors.

OK - since there's only one paragraph, it's a little hard to judge: context is always useful. But I'll nitpick it as best I can!

When a friend tells you a secret, something which they could trust no one else with, you are naturally expected to keep it, after all promises are promises.

There's one too many commas in this sentance: I'd drop the last one, and use a hypen or a colon.

However sometimes your trustworthiness can be over estimated and your head can fail your heart.

I'd change the first two words to "Sometimes, however,"

I remember when I failed my friend and did not keep my promise of secrecy.

This sentance is a little awkward, and doesn't have the easy "speaking" tone that narrative should have. Read it out loud, then try writing it down the way you would say it if you were speaking it to someone; "did not" for example, would probably be dropped for "didn't" in real life.

I had expected rage, hate and bitterness but her reaction shocked me to my very soul. Her eyes filled with disappointment, shock and most of all regret.

Perhaps the double use of shock is not especially helpful to how the paragraph flows?

Regret shall always be the most brutal of tormentors.

Shall is to out of place, to me: "Regrets are" might be better (therefore drop the 'be'); I'd also have this as an entirely seperate paragraph after the rest, just for emphasis.

Well, that's just various little bits I thought might help: hope you're able to use one or more of them.

Grammatical corrections are in bold:

When a friend tells you a secret, something which they could trust no one else with, you are naturally expected to keep it; after all promises are promises. However sometimes your trustworthiness can be over estimated, and your head can fail your heart. I remember when I failed my friend and did not keep my promise of secrecy. I had expected rage, hate, and bitterness, but her reaction shocked me to my very soul. Her eyes filled with disappointment, shock, and most of all regret. I had laughed and scorned with the crowd, but I cried out bitterly inside. Questions flooded my mind: Why? What was I thinking? How? Regret shall always be the most brutal of tormentors.

As backgroundbob said, it's very hard to judge without context, but I can say that this paragraph, alone, does not make me want to read more. It's cliche and sterotypical. It's overdone and melodramatric.

These types of paragraphs are found often in Victorian literature, but you don't find them much anymore because people don't want to start off with whining; they want a story. Showing is always better then telling.

Start off with an event. Something along the lines of, "Yesterday I told this boy, Tristan, that my best friend in the world, Clara, wasn't my best friend. I told him this because I thought he was cute and because Clara is a dead weight. To make matters worse, I added that she was a poopy head."

Starting off with an event draws the reader in. Adding something unexpected at the end also makes it humorous and keeps the reader reading.

In any case, good luck with your story!

If this is the first paragraph that begins a story of the narrator's life, which may have to do with the friend or keeping secrets, this is fine. If you want to tell a story about this secret and about the emotions you felt, then this is not quite as good and needs to be expanded.