Will the reward for my prayer be diminished by worshippers passing in front of me, due to my obsessive thoughts and my delaying to say the Taslim after the Imam?

You must say the Taslim immediately after the Imam's Taslim, as this is the Sunnah. You are not permitted to delay the Taslim after him, nor are you permitted to busy yourself with supplication or anything else after the Imam's Taslim. If worshippers pass in front of you, your prayer is valid, based on the saying of the Messenger of Allah, peace and blessings be upon him: "Nothing severs the prayer," and the saying of Ali and Ibn 'Umar, may Allah be pleased with them: "Nothing that passes in front of the worshipper severs the prayer."