You could first try looking for any relevant organisations that would consider placing a short review of the book on their website. If the website has self-help forums, then so much the better, because you can go on there and tell them about it (and chat with other people affected by mental health issues).
Also, approach your local press. I got a journalist on our local paper to write a review of my book on his local history page and made half a dozen sales from that. Not much, in the grand scheme of things, but it all helps.
Another idea is to get a small website of your own. For what you need, quite a few hosting services and ISPs offer free packages, or you could get your own domain name for just a few pounds. As for design, if you're not confident about doing it yourself and/or can't find anyone to do it for you, then type "free web page templates" or something similar into a search engine and that will return hundreds. Mostly the designers only require you to post a link back to them on your website.
Finally - and very simple, this - if you're a member of any forums where you can write a signature into your messages, then include a link to the publisher's website.