I pray you, in your letters,
When you shall these unlucky deeds relate,
Speak of me as I am; nothing extenuate,
Nor set down aught in malice. Then must you speak
Of one that lov'd not wisely but too well;
Of one not easily jealous, but being wrought,
Perplex'd in the extreme. . . .
These are Othello's last words, before he commits suicide. He seems to have regained his senses and returned to the eloquence of Act I, when he was not under the spell of Iago or his jealousy. He shows a calmness here, despite what he has done and is about to do.
We can all agree that Othello did not "love wisely." He worked himself up into such a jealous rage that he suffered seizures. He placed so much stock in the magic handkerchief (in material possessions) that he reified his wife (she too became a possession). He set her up to fail in the marriage.
The debate comes around the second half of the line, in the words "but too well." Did Othello really love Desdemona "too well"? Or did he love himself "too well"? The object or indirect object is not stated. I tend to believe the latter, that he never loved her but loved the status she afforded him. He, like Cassio, loved his repuation above all. I believe that he is trying to somehow justify his own self-delusion.
Othello's reference to the fact that he "loved not wisely but too well" can be considered in his relationships with others as well as Desdemona.
Othello is cruelly played and betrayed by Iago. Throughout the play he has never questioned the loyalty of Iago and has relied heavily on information and judgements from Iago to frame his social and political decisions. He dismisses Cassio upon the testimony of Iago (admittedly supported by the drunken Cassio) and is convinced of his wife's infidelity.
Othello needed to be in a position to trust and delegate to those around him: this would have been vital for him to rise to the military position he had. Unfortunately he had placed too much faith in Iago, and in his own judgement and allowed Iago to play on his weaknesses and, ultimately, destroy his life.