The speaker compares seeing a particular person to handling a shard of broken glass. Each encounter leaves a few small cuts, even when they are being careful. The poem never says who "you" is, but the imagery suggests someone the speaker is still drawn to, even though being near them causes pain.
What makes the poem sad is the speaker's awareness. They want to stop and say they try to stop, yet the short, flat lines show how hard that is. The last line adds a twist: trying to "piece myself back together" is the very thing that cuts them deeper. The glass seems to stand for both the other person and the speaker's own fragile state, so healing and hurting get tangled up.
The mood is quiet, tired, and resigned. There is no anger, only the feeling of repeating a painful pattern and not knowing how to break it.