I am a SEO professional. This forum ranks #2 for the search term, "psych forums", which receives 700 searches/month. The site no longer is associated with loads of mental health articles as it had been with Psych Central, which could be a reason why it does not rank for the much higher search volume term, mental health forum (which gets 6K searches/month). High quality and relevant content is a top ranking factor, which would impact the site's ability to rank for this more competitive keyword.
The site also does not target the keyword, mental health forum, in the page title tag, and the home page is missing an SEO optimized H1 heading tag, so it will be more difficult to rank for this keyword without either of those elements optimized for SEO.