I’ve a mistake. I’m suffering from waswas and I’ve made an oath saying “I swear to Allah that if I repeat my Salah or wudu due to waswas I am no longer a Muslim”. The problem is that while praying I interrupted my prayer because I’ve passed gas (90% sure of it) so I repeat wudu and salah. I’m afraid that I am not longer a Muslim because I’ve broken my oath

This type of oath has no credibility. A Muslim can be no mere muslim if he/ she denies the existence of Allah (swt) His Prophets and the day of the Judgment. About you waswas the shariya rules say if anyone has extreme level of doubts then he/she has to ignore his/her doubts.