I have this problem with hobbies: I tend to cycle through them. I might be intensely interested today, but then tomorrow, not so much. These hobbies often involve collecting things (comic books, stamps, baseball cards, books). I have learned over the years that I have to deal with these collecting hobbies in a sustainable way -- that is, don't get myself in too deep during the manic phase, so it's easier to deal with the disinterested phase.
This sustainability is especially challenging for things like comic books and stamps, which come out anew all the time and require some attention. For comic books, the best way to handle my cycling interest is to set up a standing order ("subscription") at a local comic shop, then drop in periodically to pick up my order. The key is not to subscribe to too many titles during my manic phase, because they'll inevitably pile up unread when I lose interest again. My subscription is at Lee's Comics, an excellent local store where I've shopped for about 15 years. I have a good but occasionally testy relationship with Lee's: after I inevitably ignore my comics for a couple of months, I end up in the "bad subs" box and sometimes they even threaten to kick me out as a customer. Of course, during the manic phase, I'm there every week to pick up new comics.
For stamps, I collect new issues from the United States and Åland (which as you know is an autonomous Swedish-speaking island province that belongs to Finland). For my stamp collecting, I've signed up with a wonderful place called Davidson's Stamp Service in Indianapolis. A couple of times a year, Mr. Davidson sends me my new stamps, as I run up an astronomical bill of $50 or so. When Mr. Davidson was sick, Mrs. Davidson took over. Because the stamps only come a couple of times a year, I can safely ignore them most of the time, but I still get to collect them, the perfect hobby situation for me.
My motto is "Excess, but in moderation".