First of all, ask yourself this question:
Have I submitted a story or novel to an agent/publisher/magazine?
If the answer is yes, then self-promotion isn't as difficult as you think, and there are other things that will help you grow in confidence. Start with this one:
Approach a magazine with an idea for a regular column. If you're nervous about this, remember that the worst they will do is ignore you...exactly the same as an agent or publisher. Don't start out with a weekly publication, try a monthly or quarterly one first. The advantage of this is the tagline. At the end of each piece you'll give a fifty word description of yourself and your books. Instant advertising for which you don't have to pay. It's also useful in getting you to write to a deadline. A version of this is writing book reviews.
Once you've past that hurdle, how about this:
Offer your writing services. What are you good at? Grammar, critique, editing? Proofreading, ghostwriting, cover design? This is likely to involve some financial outlay for advertising, but if you use the right places on the web, it doesn't have to cost a fortune. There will be some websites that have a relatively low number of visitors that will charge a smallish amount (£50 or so) for an advert. Start there. Stay with the genre you know and pick websites (or blogs) that you don't feel threatened by or are run by blog-buddies or twitter followers.
Now you've managed to get past these two stages you'll be ready for the final lesson:
Approaching real people. EEEEEEEEEKKK! Start small. Contact a local writer's group and offer to give a talk on writing or approaching agents. Don't charge for this (but accept a cup of tea or coffee) and get used to small groups. Once you've done this a few times you'll be more confident about approaching bookshops and asking them to stock your book or having a book signing there.
Having completed all three of these, you'll be ready for (almost) anything. What you have to do is keep on practicing, never stop with any of these things and, above all, learn what you can do and what you can't do.
If you don't try you'll never succeed. If you don't ask, you don't get.