I was angry with my friend;
I told my wrath, my wrath did end.
I was angry with my foe:
I told it not, my wrath did grow.
And I water’d it in fears,
Night and morning with my tears:
And I sunned it with smiles,
And with soft deceitful wiles.
Here the poet is emphasizing on anger management. He says that he was angry with his friend and told his friend why he was cross with him/her, and the anger did not last for a long time and vanished away. In the same way, the poet says he was angry with his enemy, but he did not tell him about it. The anger grew more because he kept it within himself. From this paragraph, the poet says that it will be easy to let the anger go away upon our friends as we talk to them regarding it, but it will be quite difficult to let it go away on the enemy, and it keeps increasing till we talk to them.

He compares his anger with a plant or a tree. We water the plant for its survival, and the author watered anger with fear, and tears day and night. We sun the plant to grow, and the poet says that he sunned his anger with his smiles and lying tricks whenever he saw his enemy and his anger continues to grow.
Meaning of difficult words:
Deceitful Lying.
State Council of Educational Research and Training (2018). Term-1 English Standard-9. A Poison Tree - William Blake (pp. 182-183). Published by the Tamil Nadu Textbook and Educational Services Corporation.