أحسن الله إليكم شيخنا If someone does something or makes a face and I accidently laugh but very quietly and hold it, then stop, is this sinful? And if so , how to stay away from it?

Laughing at something funny is not a sin, what is forbidden is mocking a Muslim or belittling him. A laugh that escaped you and that you stopped is nothing, so do not open the door of waswas on yourself.