I’m glad you wrote and didn’t keep your feelings inside. It's clear that you feel guilty for not doing what you feel was right, to save the life of your friend. That is a very understandable feeling.
No person can judge if your failure to act is a “big sin” or not. Only God knows all factors involved and can be a true Judge. Nevertheless, there is much to be learned by the difficult situation you faced, and the manner in which you reacted.
When a person is in a clinch situation like you were in, sometimes a part of their personality comes out that they weren't aware of. This is called in Hebrew a "nisayon" or "test."
It's a situation God puts a person in, to bring out parts of their personality that they might not be fully aware of. If the person passes the "test," then that good aspect of his personality gets strengthened. If he "fails," this can also lead to good, because it can be a sign for the person that he needs to work and improve this part of his personality. (The word "nisayon" also means "sign.")
Perhaps God was trying to give you a message that you need to work on your aspect of courage. Courage doesn't only mean jumping in the river. Any time you make a difficult moral choice, and anytime you choose what is right over what is convenient, that is courage. Like it says in Ethics of our Fathers (4:1), "Who is courageous, he who conquers his desires."
You mentioned you've been toying with the idea of becoming more observant. Perhaps some fears or desires are holding you back. Maybe this is the time to conquer those feelings.
