The speaker claims they can go two years without love, and the poem spends its short length explaining how. The answer is the body itself. Heart, lungs and ribs act as protective layers, a kind of storage space where love from past relationships is kept until it's needed. The title helps here: a cactus survives dry spells by holding water inside, and the speaker seems to do the same with affection.
On the surface the tone is confident, even practical, like someone describing a clever survival trick. But the confidence hides something. Needing a "protective layer" suggests that love has hurt, and the idea of living on old love suggests there may be little new love coming in. A cactus is also prickly and guarded, which fits a speaker who keeps people at a distance. The mood is self-reliant but quietly lonely, a mix of pride and a hint of sadness about having to cope this way.