In Macbeth, Act 5, Scene 3, what are Macbeth's regrets?Regrets must be for past events.

In Act 5 Scene 3, Macbeth starts out by regretting the way his life has turned out in a genericbut vivid manner. In Line 24:

                                   "My way of life

Is fall'n into the sere, the yellow leaf,.." 

The choice of the word 'fall'n" definitely signifies regret or some kind of failure. The metaphorical use of 'the yellow leaf ' indicates a downward turn of events that can only usher in death literal or figurative as in the fallen yellow leaves of the autumn season.

Macbeth also regrets that at this point, he cannot and will not age gracefully. He knows what  the  most important qualities of a happy, health old age are per Line 27:

' As honor, love, obedience, troops of friends,'

  Per line 28, he knows that these that these qualities will not be available to him.  He knows with with deep almost embittered regret what is in store for him as in Line 29:

'Curses not loud but deep, mouth-honor breath,'               

His choice of words as he describe the 'Curses...' is quite significant. In 'not loud but deep, mouth-honor, breath,..", the description of the 'Curses' is so vivid that the audience can almost feel their impact which is definitely regrettable. The regrets climaxes and comes out with cutting clarity in Line 30:   

'Which the poor heart would fain deny, and dare not.'

Use of term 'poor heart' signifies  pure regret of past events and 'fain deny' his desire to deny the regrettable events. He still has enough presence of mind, however, to admit that much as he would like to, he cannot deny the past any more than he can erase it