Wingy
It has to be remembered that it was Winslets character that started the ball rolling by erasing her memories of Carrey from her mind. He responded (in an understandable fit of pique) by asking for the same service to be done for him. However having started the procedure he remembers just why he fell in love with her in the first place, and from that point on it is a journey of travelling back through the memories of the relationship from the good to the bad and examining why it went bad, why she left him, and why she would take the extreme step of having him erased from her memories.
The sweet, sad and disarming aspect to the movie is how Carrey's mind invents a sympathetic version of Winslet in his minds eye which naturally, because it is a figment of his imagination, is prepared to assist him in trying to save the memories of their relationship in a way that the real Winslet, at that point, probably would not have done. The Winslet he creates in his minds eye allows him to have the kinds of conversations with 'her' as they examine the memories of their relationship that he probably wished he could have had the opportunity to have had with the real her in real life.
Again, I think most of us have been at that situation when, picking through the pieces of a failed relationship that we didn't want to end, wish we could go back through time to a particular point in the relationship where we could have had the opportunity to choose a different path or even have the opportunity to say the sorts of things about what we were thinking and feeling at the time, but for some reason didn't say, to our eventual cost.
In the movie, Carrey gets the opportunity to have those sorts of conversations with the 'fictional' Winslet as they wade through the debris of his memories, which provide him with a sense of closure and relief from his personal demons that he would have got had he had been able to have those sorts of conversations with the real Winslet at the end of their relationship. It is of course another layer that adds to the psychology of the movie, that these 'conversations' are in effect a monologue between himself and his vision of Winslet as he would have liked her to have been. A vision of Winslet, which over time, when reviewing the memories of why they split, becomes increasingly distant from the behaviour and motivations of the 'real' Winslet of his memories.
Having gone through this process of reliving and dealing with even the most painful of memories it allows him to, in effect, start over with the real Winslet at the end of the movie with a relatively clean slate, perhaps wiser and more able to make a better go of it the second time around. However human nature is a powerful and deterministic thing and I like how the movie leaves it moot as to whether the two of them have a real chance to make it the second time around. I think there are grounds for both optimism and pessimism in this particular case.