The more exposure I have to psychology (through my therapist friends) and the more I study magick, witchcraft, and astrology, the more similarities I see between the two. They are both, after all, trying to figure out the heart/emotions, mind, and spirit as well as figure out ways to work with them and cause intentional changes in them for their healing and betterment.
Being modern and secular, and to maintain its air of respectability and broad applicability, psychology eschews topics that might touch spirituality, such as defining how the spirit and or soul fits with the mind, which is probably for the best as that is individual.
Self-help is the popularized monetized secular equivalent to chaos magick: whatever works: glitz it and ship it!
Self-help pilfers from psychology that which it finds useful enough that they can sell to people for the effect promised.