Sam M's "I'm sorry" is an apology that refuses to be a full apology. The speaker has started something with a man who used to be with someone close to them, and that person had asked them to stay away from him. The poem reads like a confession addressed straight to that person, and it keeps splitting every "I'm sorry" from a "but I'm not sorry."
The speaker feels guilty about the hurt, the broken trust, and not being honest earlier. Still, they won't take back the choice itself. The line about the man destroying the other person while building the speaker's heart is the most honest and uncomfortable moment, because the speaker's happiness comes out of someone else's pain.
The mood is nervous but determined. The speaker admits this could end in love or heartbreak, even that he might stop talking to them tomorrow. What matters is that for once they followed their own wishes, not someone else's.