Praise be to Allah.
You have done well to call these thoughts that cross your mind waswas. It is well known that waswas comes from the Shaytan, because the Shaytan does not want anyone to turn back to his Creator in repentance, regretting (past mistakes) and turning towards what is right and good. Rather, he wants to divert people away from religious commitment using all kinds of temptations and specious arguments. If he is unable to do that, he resorts to waswas and stirring up doubts, to make a person feel anxious and uncertain. Hence you will find that you will not experience this waswas except after you have given up some sins that he used to make appear attractive to you. When you prevailed over him in this area, he resorted to the weakest of his tricks, which is waswas. The Companions of the Messenger of Allah (peace and blessings of Allah be upon him) complained to him that they noticed some waswas that they disliked and did not want to speak of, and he (peace and blessings of Allah be upon him) said to them: “Praise be to Allah Who has reduced his tricks to whispers.” Narrated by Abu Dawood from ‘Abd-Allah ibn ‘Abbas ; classed as saheeh (authentic) by al-Albani as stated in Saheeh Abi Dawood.
When he was unable to stop them doing good, he resorted to waswas.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Waswas is something that happens to everyone who turns to Allah in dhikr (remembrance) etc; it will inevitably happen to him. So he should be steadfast and remain patient, and persist in his dhikr and prayer, and not give up, for by persisting in it he will foil the plot of the Shaytan and ever feeble indeed is the plot of Shaytan (4:76).
Every time a person wants to turn to Allah with his heart, whispers of other matters come to him. The Shaytan is like a bandit; every time a person wants to travel towards Allah, he wants to intercept him. Hence it was said to one of the salaf (pious predecessors): The Jews and the Christians say, we don’t experience waswas. He said: They are right! What would the Shaytan do with a ruined house?
Majmoo’ al-Fatawa, 22/608
Hence, you should not pay any attention to this waswas, and do not let it form an obstacle on your way towards Allah.
With regard to what you have mentioned about some of the kuffar (non-Muslims) and innovators enjoying a sense of happiness and tranquillity, you did well to say in your question, “but I do not know if it is real.” Many of these pleasures are false, appearing outwardly whilst inwardly the person feels lost and anxious, which can only be dispelled when a person is truly sincere towards Allah in submitting to Him and pleasing Him.
Here you should pay attention to a number of matters:
1 – The measure of sound ‘aqeedah (creed) is not whether one feels at ease or not, rather sound ‘aqeedah is known from what is stated in the Quran and the Sunnah (prophetic tradition) of the Messenger of Allah (peace and blessings of Allah be upon him), in accordance with the way of the Companions of the Messenger of Allah (peace and blessings of Allah be upon him). If you are confused about any matter, measure it against the words of Allah, may He be exalted, the Sunnah of His Messenger (peace and blessings of Allah be upon him) and the words of his Companions. If you find that the Companions of the Messenger of Allah said that, then you will know that it is true, and anything other than that is false. If you are unable to do that, then ask scholars whose knowledge and method are in accordance with the way of the Sahabah (Companions) and the righteous salaf (predecessors). This is the only true standard.
With regard to the happiness and tranquility that are the result of sincerely seeking the pleasure of one's Lord and following the Sunnah of His Prophet (peace and blessings of Allah be upon him), Allah says (interpretation of the meaning):
Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life (16:97)
2 – Feelings of anxiety and pressure are something relative; it varies from one person to another. Sometimes a person may live in the most difficult of circumstances and not experience any such feelings, because he is dead at heart. Do you not see how a blind man may live in the most intense darkness but he does not feel that it is dark? That it because he does not have any sight at all. The same applies to one who is dead at heart; he does not have any feeling of life at all, so he cannot feel a sense of loss at its absence.
But Allah says, and His word is true (interpretation of the meaning):
And whoever turns away from My remembrance - indeed, he will have a depressed life, and We will gather him on the Day of Resurrection blind." (20:124)
The word “dank” (translated above as depressed) has been interpreted in a number of ways. In the Tafseer of Ibn Katheer (may Allah have mercy on him) it says:
i.e., depression in this world, so he will not feel any peace of mind or ease, rather he will feel constricted because of his misguidance, even if he seems outwardly to be enjoying the luxuries of life, wearing what he wants, eating what he wants, living wherever he wants. But his heart has not found certainty and guidance, so he is in a state of anxiety, confusion and doubt, and he will remain in that state. This is the life of depression.
Al-Dahhak said: it refers to evil deeds and provision that comes from haram (unlawful) sources.
It was narrated that Abu Sa’eed said, concerning the phrase “a depressed life”: His grave will be constricted for him until his ribs interlock.
It was narrated from Abu Hurayrah that the Prophet (peace and blessings of Allah be upon him) said: “indeed, he will have a depressed life refers to the torment of the grave.’” Narrated with a jayyid isnad (good chain). End quote.
If we assume that a kafir (non-Muslim) or evildoer lives a happy life in this world, even inwardly, he is still lacking in the happiness and tranquility that the true believers enjoy, and what awaits him of torment in al-Barzakh (life in the grave), and thereafter is terrible hardship. We ask Allah to protect us and you from the torment of the grave, and to make us steadfast in adhering to the truth until we meet Him.
Finally, you should strive to do acts of worship and good deeds, and shun the whispers that cause anxiety. You should acquire beneficial knowledge, for it will protect you, by Allah’s leave, from all sorts of confusion and uncertainty.
And Allah knows best.