The speaker is trying to break free from someone who has taken over their emotional life. The poem opens with a simple, almost desperate statement, "I need to be me," and everything after it explains what is getting in the way.
The two middle lines are the most striking. When the speaker is sad, the other person's tears are "drowning" them, so even grief isn't their own. When the speaker is happy, "nails clawing" at their body turns joy into something painful and possessive. Both images suggest a relationship where the speaker can't feel anything freely, though the poem never says exactly who this other person is.
The last line, after the pause of "But most of all," changes the tone. It moves from complaint to goal: the speaker wants to learn to exist without this person. The mood is raw, tense, and tired, but there's also a quiet determination in how plainly the speaker asks for what they need.