Fact: The sheet music publishing world is tough to break into. I have been writing choral music for nearly 20 years and have yet to find a publisher that is willing to take a risk on me. When I get the "letter" from publishers, I'm usually told that they don't have time or resources to give individual feedback for each piece submitted, but that what I sent them was not what they were looking for. That leads to the question, "What are they looking for?"
Fact: Breaking into sheet music publishing is more about who you know and good timing than having quality, publishable, performable pieces. I know my compositions are able to be performed because I have performed them. I know that audiences enjoy them. I just don't seem to have the knack of getting my pieces to the right publisher at the right time. And I don't know any of them personally so I'm always making a "cold sale" when I approach them - a difficult proposition for all but the most experienced sales person.
Fact: Publishing through a big name publisher doesn't make very much money for the composer. Part of each sale is divided up between publisher, retailer, and sundry other people before the composer (and possibly lyricist) get their small cut.
Myth: There's nothing you can do about it.
In the world of technology that we live in, it has become increasingly easy to self-publish your music. There are website hosting services (such as shopify.com, squarespace.com, or weebly.com among others) that can help you set up your own online store. There's usually a monthly or annual fee for these services, so choose wisely. Pros: you get to keep the money from your sales. No one else is taking a chunk from the price you set. Cons: if you're not having a lot of traffic and a lot of people visiting your site and buying your music, you might end up paying more for the site than you are making from your music.
There are other options out there as well. There is a great site out there that I use called CadenzaOne.com where any composer can upload their music for free, set their own prices, and make it searchable on their site (and in effect, on Google). They take a very small portion of the sale price to offset the costs of running the site, but besides that, they money goes directly to you. Pros: Your music is out there with a URL that you can share with others. It's a fully searchable site. The sales are taken care of by someone else, but you get to keep most of the money. Cons: Your music is up against a lot of other music from other publishers and self-publishers. It still takes a lot of time to market your own music.
Fact: However you choose to self-publish, you're going to have to work hard to get people aware that your music is out there. You don't have a publisher doing the work for you. Currently, I have about 40 songs listed on CadenzaOne, I've created this webpage where I can link to them as well as post other songs that I'm offering for free (for various reasons), I have a Facebook page (@sdcompose) where I can let people know about my latest compositions, and I am a member of several choir and composition groups where I can also make directors aware of what I have available (including ACDA Choral Composition Initiatives). There is also a heavy ground game that involves talking to choir directors that I am familiar with and showing them my pieces. The more you can get your pieces performed, the more likely it will be that other conductors will be interested in performing your pieces.
Fact: I, and most composers, don't compose with the idea of publication in mind. So, keep composing and don't give up!