Four short lines, and the whole poem rests on one comparison: moving on feels like being able to breathe at last. The word "finally" matters here. It suggests the speaker has been holding their breath for a long time, stuck in something painful, whether a relationship, a loss, or a bad stretch of life. The poem doesn't say which, and it doesn't need to.
The speaker feels relief, and a little joy. The line "And dear God" reads like a spoken sigh, as if the thought slipped out before the speaker could polish it. Then comes the simple, almost cheeky ending: "Is the fresh air nice." There's no grand statement, just someone enjoying a good feeling.
The mood is light, honest, and refreshing. Because the language is so plain, the relief feels believable rather than dramatic. It's the kind of thing you might say aloud once you realize you're okay again.