Firstly: the best remedy for waswaas is to ignore it and not pay any attention to it, and not worry about the doubts that the shaytaan may cast into your mind about purification, prayer etc.
And you should also say a lot of du’aa’ (supplication)and ask Allaah to keep you safe and sound, and seek His help against the tricks of the shaytaan.
See the answer to question no. 62839.
The basic principle is that a person is still pure (taahir) and he does not have to do ghusl unless he is certain that he became junub. So long as he is not certain then he does not have to do ghusl, even if he thinks it most likely that he became junub.
A man who thought that he felt something whilst praying complained to the Messenger (peace and blessings of Allaah be upon him) and he said: “Do not stop until you hear a sound or notice a smell.” Narrated by al-Bukhaari (137) and Muslim (361). What is meant is: until you are certain that you have broken your wudoo’(ablution).
It seems that you do not have to do ghusl so long as you are not certain that you have become junub.
And Allaah knows best.