I do feel as if it is harder for me to feel creative when I have been depressed or under a lot of stress. A major part of depression is a loss of interest in things, and interest is the core of creativity, so it stands to reason that you would have a difficult time being creative as a result of that depression.
I have taught art to adults, and I think creativity is like anything else--if you fall out of the habit of it, it is difficult to get back into the habit of thinking creatively. Try taking a few workshops if you can, on techniques that are specific and focused rather than vague (for example, choose a workshop on "multi-media journal making" rather than "the basics of watercolor"). Experiment with one project; then do a variation of that project; then a few other projects based on further variations; etc. It is amazing how much easier it is to generate ideas if you start with one idea and see how far you can take it. For instance: you might know of an amazing view that you would like to paint. So get up really early for several days and paint it at sunrise. Then, over the course of another week, paint it at noon. Then go back for a third week just before sunset. Three paintings (or more!) from a single idea! In my experience, this approach is excellent for rebooting your ability to think creatively, because you are approaching each idea you have from a variety of angles, and as this becomes habit, you'll generate lots of ideas more readily.
Also, check out any book by Eric Maisel, a noted creativity coach: Brainstorm, The Van Gogh Blues, etc. They are awesome and his techniques are very effective.