Unsettled

Last year a friend of mine told me about 2 our friends who was dating... the matter was that the girl had a boyfriend since quite an year. The boyfriend is the brother of a friend of mine (he is like my best friend) who I'm very related to...
I'm keeping the secret since an year... I couldn't tell him because I'd got in trouble the friend who told me, and telling would meant to create a big big mess between all my friends! and specially I was scared of the reaction of my best friend... I thought he would be angry because I didn't immediately tell him, as soon as I knew...
Anyway... someone found out the guys were dating and now my best friend told me about the "bad new" which he has now known! When he was telling me I pretended to be surprised and scandalized.... but I feel like a lier!
I don't know if it's time to tell all the truth or not... I think I'll keep not saying anything, and if he one day will find out that I knew something I'll let him read this page.... and I hope he will forgive me...