Must I deal with the intrusive thoughts that I could have done more to prevent the divorce, and what should I do about it?

Whispers [of doubt] contain no good, and one must ignore them and not follow them.

In Sahih (definition) Muslim: "The strong believer is better and more beloved to Allah than the weak believer, and there is good in both. Be keen on what benefits you, seek Allah's help, and do not be helpless. And if something befalls you, do not say, 'If only I had done such and such, it would have been such and such,' but rather say, 'Allah has decreed, and what He wills, He does.' For indeed, 'if only' opens the door to the work of Satan."

Ibn Taymiyyah said: "He (the Prophet) commanded him to be keen on what benefits him, to seek Allah's help, and forbade him from helplessness and reliance on fate [without effort]. And he commanded him not to despair over what he missed, but rather to look to destiny and submit the matter to Allah."

What benefits you is to consider your matter carefully, consult with wise people, and look at what is most beneficial for you. If it is most beneficial to return to your wife and the divorce was less than three [talāqs], then seek Allah's help, perform Istikhara (prayer for guidance), and take her back. But if it is most beneficial to separate from her, or if the divorce was three [talāqs], then do not preoccupy yourself with her. Seek a righteous wife and turn the page on the past.