Seneca’s third letter. On Friendship.
These few bits are all you need on this:
// Although you should live in such a way that you trust your own self with nothing which you could not entrust even to your enemy, yet, since certain matters occur which convention keeps secret, you should share with a friend at least all your worries and reflections.
// It is equally faulty to trust everyone and to trust no one.
And onto the topic of focus:
// For love of bustle is not industry, – it is only the restlessness of a hunted mind.
QED:
// Discuss the problem with Nature; she will tell you that she has created both day and night.
I have nothing to add.