I fed milk products to my vegan friends
Why would you feed your friends things they don't want to consume?
I forgot what was in it then I realized and just kept quiet.
Even if you don't understand your friends' dietary choices, it is wrong to deceive them into consuming products they don't want to be eating. You should respect their desires in the future.
I was sorry.
You need to be more careful in the future and don't tell them what you did. You could make them feel like you broke their trust.