Here we see yet another contrast between different things based on how long they last in time. First, the speaker says that he wants to be "stedfast," which we can take to mean "stedfast [like the star]." (The fact that he is repeating the same word from the opening line, when he compared himself to the star seems to point in this direction, don't you think?) So, he's saying he wants to last for all time – but then, he wants his head to be "Pillow'd" on his girlfriend's "ripening breast."
Now, the word "ripening" probably has two main meanings here. First, it suggests that the speaker's girlfriend is still fairly young and so is still in the process of "filling out." The second thing is that the word "ripening" gives us a feeling of sweetness and warmth that contrasts with the cold images of the "moving waters" and the snow falling on barren landscapes from earlier in the poem. But does this image only contrast with them – or does it have some similarities with those images as well?
We think it very well might have some similarities with them; if it does, there would be a third meaning to the word "ripening" in the context of this poem. If the reference to "new soft-fallen masque / Of snow" calls to mind the changing of the seasons, and thus the passage of time, it's hard to see how the image of "ripening," which happens to fruit in the autumn, doesn't do the same thing. So, really, what we have here is yet another image of an eternal thing (what the speaker imagines himself being) and a thing with an expiry date: the "ripening" breast of the girl. Why a thing with an expiration date? Well, doesn't ripe fruit eventually turn all wrinkly and then rot?
"But wait," you're probably saying, "Keats doesn't say anything about the girl's breast turning wrinkly and rotting! And anyhow, how could he keep his head on her breast forever if her breast doesn't last forever?" And you know what? If you do say that, we agree with you completely. The eternity of the girl's breast, and therefore of the girlfriend, really does seem to be part-and-parcel of what the Keats's speaker wishes for at the end of the poem here. So why does he say "ripening"?
Well, we think it's almost like a kind of reverse psychology move – as if Keats is saying, "Yes, ordinarily this breast would ripen and keep ripening until the woman dies, but in the world I want to live in, the world of my imagination and desire, it can keep ripening forever, and I will keep my head there forever." But if this is true, then it looks like we have to change what we said before – that this is "another contrast between different things based on how long they last in time." Actually, it's starting to look like Keats's speaker is talking about two things that both would ordinarily rot (the speaker and his mortal girlfriend), but which, through some miracle – if he could somehow have the "stedfastness" of the Star – will not rot, will not be subject to time. Pretty cool, don't you think?