How can a new revert get rid of remembering past sins, overcome the distress caused by associating with bad company at work, dispel doubts and illusions about his fiancée, and strengthen his faith and devotion in prayer?

Recalling sins and regretting them is praiseworthy, but if it reaches the point of obsessive doubt (waswasah), it becomes a trick from Satan. And doubts about your fiancée are from Satan's tricks, so do not pay attention to them. The default for a Muslim is safety (from sin) and thinking well of others. If the girl is religious and of good character, and you are able to marry, then hasten to complete the marriage, as it helps in preserving one's chastity and staying away from temptations. Be wary of friends and distance yourself from them if possible.