I believe "Lumps" comes from the phrase "Taking your lumps" as in, making a mistake to grow into a better person. Having that experience doesn't make things easier to deal with all the same though, as is evidence from the song, where he's wishing he hadn't screwed things up and earned these lumps. It's kind of said in a cathartic way. As in, "the bright side of this learning experience is that I've fucked up the best relationship I could have ever had, but at least I know better now".
I believe "Lumps" comes from the phrase "Taking your lumps" as in, making a mistake to grow into a better person. Having that experience doesn't make things easier to deal with all the same though, as is evidence from the song, where he's wishing he hadn't screwed things up and earned these lumps. It's kind of said in a cathartic way. As in, "the bright side of this learning experience is that I've fucked up the best relationship I could have ever had, but at least I know better now".