There are a lot of good suggestions posted. I'd suggest looking at some of the major job engines and their other services like "templates" for cover letters and resumes. They specifically address issues such as a long gap in employment or other issues.
You can always use a good search engine to ask the question, "How do I explain a large employment gap in my resume?" You may be surprised how many resources are available.
I totally agree with the suggestions above and I think you'll find the experts at writing resumes who give the templates for free also emphasize, that ever sentences focuses on your abilities now and how you learned, networked etc. during the time you weren't employed that will benefit them.
Remember, you have to first sell yourself like everyone else does that you can HELP an employer solve certain problems. Start thinking first of what are your strongest skills and most comfortable for you. Then think of what type of employers need your skills.
Basically, keep your resume to the resume page and your brief explanation of why you were out and how you've resolved the issue and learned x,y, z during that time to make you an even better candidate for the employer.
It's a really tough call when it comes to mental health issues, partially, b/c I don't know if you'd need security or other clearances, where truly having those conditions could endanger you and others if you couldn't perform at your peak. If you feel you have the conditions under control you can always write something like now that my medical issues have been appropriately treated, I'm looking forward to....... or something similar.
Check the net. There are a lot of really good resources for how to deal with special situations in an application and resume/cover letter. I hope you'll use them.
MSR