It is unfortunate that your friend passed away before you were able to do the things that you wanted to with your friend.
Grief can be difficult, it is personal and sometimes it is a difficult burden to put down.
Remember that your friend valued you and the things that you did together and thats why they chose you as their friend.
Take the time you need to remember the good times and the bad times. Plant a tree, share some memories, write a letter go for a walk. There are many ways that grief can be expressed when you are ready to stop blaming yourself for not knowing something that you have only just found out about. hugs