Most Songwriters write songs for the wrong reason. I said wrong reason, in the singular. They think that every song they write has to be part of an album of their band, or one of their so called signature songs in order to make money. Then when they aren't successful at first they get frustrated and don't write as much. Then if an album isn't successful or they get bad reviews they literally give up because media or other musicians don't like their songs. You have to think differently than that.
Or you could take up Shark Petting in the Indian Ocean. Tough choice I know, it's a head scratcher.
Write songs to gain experience and offer to write songs for free on your band website. Don't even try to sell them as Mp3's. That's defeating the purpose of songwriting when you first start writing songs. Do it mainly to learn as you go along in your career as a songwriting musician. If you really are a dedicated writer you won't be able to stop writing songs. As Canadian Bass Player Mike Nelson said about Pete Denomme... "Pete's a Songwriting Machine!"
Thinking Differently About Marketing Your Songs By Pushing The Free Line:
What I mean by that is offer your songs for free on your band website to your fans and other musicians. You'll develop fans more willingly and organically if you offer your songs for free at first, and fans won't think you're just trying to sell them something. As well, offer to write songs for other bands for free. What? You're probably seething and frothing at the mouth now, especially the musicians who actually make money as songwriters. Yes, offer to write songs for free. Don't worry if a band actually uses it and makes money from a song you wrote for them. So what. The more artists you help by writing free songs for them the more experience and songwriting chops you'll gain as a result. Start approaching record labels and songwriting groups to learn what is involved in writing a marketable song.
Eventually become part of Songwriting societies that protect your songs like The American Society of Composers and Publishers (ASCAP) and The Society of Composers Authors and Publishers(SOCAN) in Canada. Keeping on pushing the free line offering to write free songs for studios who need songwriters.
But songs on your band's CD's are keepers and should be protected. Learn all you can about royalties, protecting your songs and keep pushing the free line.