There's "online gambling" and then there's "online poker". They are two COMPLETELY different things.
If you are addicted to online gambling (I'm assuming virtual casinos
) then, yes, you have a problem because you will lose in the long-term no matter what system you use.
If you are addicted to online poker then it depends. If you play to the well-known tried and tested formulas and use patience and discipline then you do not have a problem because you will win long-term.
If you are addicted to online poker and treat it as if it's gambling and have no logical way of playing then, yes, you have a problem.
Does online gambling cause mental stress or mental problems? Again, if you are referring to virtual casinos then I'm sure that for many people this would be an unfortunate "yes".