I don't think there's any relationship in the world where there's not going to be a SMALL part that's at least A LITTLE relieved it's over, she's not even necessarily wrong about THIS relationship but in that case I think it's more luck than any actual insight.
There hasn't been a single relationship I've ever had end that I didn't find some measure of relief after it was over. The ones where you end it yourself on your terms are just the ones where you find that relief a lot earlier on. However, even the most heartbreaking ones will find you eventually seeing that all the other person did was save you the trouble of waiting so long to see the truth: it just wasn't ever going to work out. They just saw that was the case before you did.
While I can never condone of what she did that brought the inevitable break-up, I do believe that it's possible that Mel and Andy just weren't compatible with each other. It always sucks when one realizes that, even when you love someone so much. I'd like to think that even if Kara was wrong, she's saying what Mel needs to hear to pull herself together and walk her life in confidence again.