'This is not what you think' Is there any way to frame this sentence without using 'not' or any other words meaning the same

You would need to come up with sentences meaning the same as this without actually using not or similar words/synonyms and actually the sentence shouldn't be showing any negative vibes or emotion

A person uttering This is not what you think presumably knows what the other person is (likely) thinking and should be able to reformulate the contradiction as:

You may be thinking this is _____; however, it is really/just/merely _____ .

or

You may be thinking this is _____, but isn't it really/just/merely _____ ?

Rather than contradicting the thought by saying only what something isn't, the therapist can state the (presumably) less serious/disturbing (or more accurate) reality of what it is, drawing the patient's attention away from the erroneous thought by offering a more realistic/appropriate alternative. In addition, the reformulation gives the patient a chance to confirm or correct that therapist's assumption.