“Life is overwhelming” is correct - drop the article. “And does it mean"life is difficult and stressful"?” It could mean that, yes, but you can’t know for certain. One meaning of “overwhelm” is to be emotionally very strongly affected. It can be both positive and negative: “Joy overwhelmed her when she realized that she had won a million dollars.” “He was overwhelmed with guilt.” Other meanings of “overwhelm” are: “to cause (someone) to have too many things to deal with” and “to defeat (someone or something) completely” So “Life is overwhelming” can have several meanings.