I am a young man and I am suffering from a mental illness which I think is waswas (insinuating whispers). Some days I wake up and I am not sure whether I have become junub (in a state of ritual impurity) or not. I looked in my underwear and thought that I was junub, but I did not pay any attention to it and I did not do ghusl (bathe). But as the day wore on, the waswas started. What should I do?

Praise
be to Allah

Firstly: the best remedy for
waswas is to ignore it and not pay any attention to it, and not worry about
the doubts that the shaytan may cast into your mind about purification,
prayer etc. 

And you should also say a lot of
du’a (supplication) and ask Allah to keep you safe and sound, and seek His
help against the tricks of the shaytan. 

Please see the answer to
question no.

62839. 

Secondly: 

The basic principle is that a
person is still pure (tahir) 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
Allah be upon him) and he said:  “Do not stop until you hear a sound or
notice a smell.” Narrated by al-Bukhari (137) and Muslim (361). What is
meant is: until you are certain that you have nullified your wudu
(ablution). 

It seems that you do not have to
do ghusl so long as you are not certain that you have become junub. 

And Allah knows best.