Phil,
Your physical malady of Pink eye is as good a theory as any. I have read the different opinions and views that have been presented on Paul's thorn. As described as a thorn in the " flesh", one usually must presume that it was a physical ailment. However, there is one possibility that has not been mentioned. What if Paul's "thorn" was a heart problem ? We are all sinners saved by grace, but we all are born with the sin nature of Adam, a fallen nature. "The heart of man is deceitfully wicked above all things, who can know it " ? Paul even writes that he was "chief" among all sinners.
While disease and infirmity makes for a case, i submit that Paul may have suffered from a "heart" condition of guilt and doubt. We all have it. We are surrounded by a sea of unbelievers and opponents just as Paul was. The devil being the greatest of these, he is able to "sift us as wheat" just as Peter was. What is dredged out of the depths of our wicked hearts is fuel that the enemy can use to cast doubt and uncertainty and create guilt about "what could have been" or "this proves you are unworthy to practice what you preach"..thoughts that are injected to condemn and sideline us.
Paul, as human as any of us, was in the possession of a great amount of information and knowledge of the truth.  "The flesh warreth against the Spirit..." This knowledge can be maddening because the flesh has limitations that require we lean and depend on The Lord for direction and renewal to persevere. But Paul obviously never got a break or vacation. He was beaten down and locked away. King David even crumbled and begged the Lord when all seemed to encompass and everything appeared grim.
Also as the Lord was transitioning to " my GRACE is sufficient for thee..." from " signs and wonders follow those who believe ", We must realize that full grace has it's benefits and it's negative aspects.
There is also the factor of others that will either support us or cause us trouble. We are not to lean on the arm of flesh. People will either " make us or break us"..we can pray for them, but we cannot control them. Paul no doubt was concerned that all he had accomplished was being subverted by rogue elements. He was willing to himself to be accursed for his brethren's sake.  Frustration can harden the heart.