The speaker is talking directly to someone, "you," who showed them two very different sides of love. The first lesson is short and gentle: love can be amazing and beautiful. The poem's few words and quiet lines make that sentence feel almost like a memory of a happier time.
Then the word "but" turns everything around. The second lesson is longer and heavier. Love also brought sleepless nights, soft crying close to midnight, and the question of how much pain a person can take. The detail of crying "softly" suggests the speaker hides their hurt, perhaps so no one else will hear.
The mood is sad, tender, and a little tired. The speaker doesn't sound angry. They seem to be looking back and admitting that the person who gave them joy also gave them real heartache, and that both things are true at once.