Unlawful matters are the things that trouble man’s mind in the sense that one is never at ease in doing or getting involved in them or, as the Prophet put it in the hadith you quoted, “that one feels embarrased when people get to know about it.” But as for “WASWASAH”, it refers to insinuations which Satan puts in man’s mind with the aim of driving him away from the Right Path.
Generally speaking, Islam has clarified for us lawful and unlawful matters. When one is in doubt about the exact ruling of something, then, one should resort to a faithful and reliable scholar who will give the answer in the light of the Qur’an and Sunnah of the Prophet (peace and blessings be upon him).
However, one sometimes doubts lawful matters. This is due to the whispers of Satan to confuse one and to make doubts creep into one’s mind, in order to distort one’s faith; this is what you name in your question as “WASWASAH”.
Surely, a Muslim must abide by the command of Allah and His Messenger and apply them in life. Thus, he/she should keep his/her faith intact with regard to the issue of Halal and Haram. However, one should not inquire into things that are not of one’s concern. This causes Satan to overwhelm one and may induce one to go astray. One should give up thinking in matters that do not concern one. A good Muslim steers clear of dubious matters.
It’s worth mentioning that the Jurists say that a person should adapt his/her soul to seeking the truth, otherwise he will find himself indulging in wrong things. Thus, a Muslim who is intelligent and aware of his needs should stick to priorities. In this way, he has to make use of his time. If he doesn’t do so, he may end up wasting his time on trivialities.
That is why the Qur’anic and Prophetic guidelines constantly urge Muslims to occupy their time seeking effectual knowledge and accomplishing good deeds. Once, the Companions asked the Prophet (peace and blessing be upon him) about the new moon, Allah, Exalted and Glorified be He, informed them the wisdom behind it saying, “They ask thee concerning the new moons. Say: They are but Signs to mark fixed periods of time in (the affairs of) men, and for pilgrimage…” (Al-Baqarah: 189).
Anas (may Allah be pleased with him) reports that once a man came to the Prophet (peace and blessings be upon him) and asked him, “When will the Hour be, Messenger of Allah?” In responding to the question, the Prophet (peace and blessings be upon him) told the man that the thing he should concern himself with is how to prepare himself for this Hour rather than inquiring about it.
So, as a muslim you should try your best to steer clear of things that fill your mind with confusion. Whenever the whisperings of Satan occur to you, you must turn your attention immediately away from them by seeking refuge in Allah (i.e. by saying: a`udhu billahi mina ash-shaytani ar-rajim; I seek refuge and protection in Allah from Satan, the accursed). Try as much as possible to adhere to the Glorious Qur’an, reading its verses and learning its meanings. You also should learn more about the Prophet (peace and blessings be upon him). In this way you will, insha’Allah, find the outlet and remedy for such Satanic whispers. May Allah help us all.