I think "
Of late, I think of Cliffordville
" might fall into this genre. Only in Ciffordville, instead of a guardian angel, it is a ravishingly beautiful devil (Julie Newmar). The alternate reality has him going back in time and, in effect,
reversing
the main character's success. It is sort of a darker version of
It's A Wonderful Life.
A World of difference
also falls into the alternate reality genre where the main character thinks he is one person, but really is another. He retreats to a happier alternate reality.

Never say never

Honestly, when I first read your question the first episode that came to mind was "The Changing of the Guard." A man who thinks he's contributed nothing to the world is shown people from his past and comes to realize he led a wonderful life after all. I was surprised it required a clarification
Honestly,
brimfin
, I had completely forgotten that the setting of that ep took place on Christmas Eve ! The fact that the ep's plot also liberally borrows from
Goodbye, Mr. Chips
also gave me pause.
But
Jennie_Portrait's
capsule summary clarified things completely for me, and carried the day.
"A Passage for Trumpet" Man in poor straits contemplates suicide, is shown that his life is worth living by an angel.
I would accept that.
"Person or Persons Unknown" Like George Bailey, David Gurney finds that people he knows well no longer know who he is. (Bit of a stretch there, really)
Spot on,
brimfin
multiple parallels there not only do Dave's wife, friends, and co-workers no longer recognize him he's also at a bar, contemplating his fate, chatting with the bartender while trying to resolve his dilemma there's a scene where there's a small crowd, and he invokes each person by name, asking / begging them to remember him the police get involved and most pertinently, there's a heart-wrenching scene when he contacts his mother, who of course,
has absolutely no recollection of his existence !
Even "Zusu's petals" disappear, in the form of his photograph with his wife.
Total 6 pts
Nice work,
brimfin
!
One last TZ ep that definitely has many parallels to
It's A Wonderful Life
and, I'm sure, a few others that bear strong resemblance !
J'ai l'il AMRICAIN !

Honestly, when I first read your question the first episode that came to mind was "The Changing of the Guard." A man who thinks he's contributed nothing to the world is shown people from his past and comes to realize he led a wonderful life after all. I was surprised it required a clarification.
My next guess after that was "Cavender is Coming" and then "Mr. Bevis" which involved guardian angels. ("Cavender" is already taken now, of course.)
Other guesses:
"A Passage for Trumpet" Man in poor straits contemplates suicide, is shown that his life is worth living by an angel.
"The Trouble with Templeton" a man is allowed to interact with characters from his past, finds them not as he remembered them (in TZ because they were acting different, in IAWL because their histories had changed without his contributions.) He ends up satisfied with the life he has.
"Person or Persons Unknown" Like George Bailey, David Gurney finds that people he knows well no longer know who he is. (Bit of a stretch there, really)
Any matches?

I can't help but think of the episode, "Walking Distance".

This message has been deleted.

In Changing of the Guard, the main character contemplates suicide. He is in despair because he feels that he has not caused an impact on the world. It takes place on Christmas eve. He realizes via students coming back from the past that he has indeed had a wonderful life, helping to influence young minds in making the world a better place.
That would do it, Jennie_Portrait.
+3 pts.
J'ai l'il AMRICAIN !

Please share a little more information as to why you feel Changing of the Guard bears strong similarities to It's A Wonderful Life.
In
Changing of the Guard,
the main character contemplates
suicide
. He is in despair because he feels that he has not caused an impact on the world. It takes place on
Christmas eve
. He realizes via students
coming back from the past
that he has indeed had a
wonderful life
, helping to influence young minds in making the world a better place.

Never say never

Mr. Bevis
Cavender is Coming
Changing of the guard
Nice work, Jennie_Portrait
Cavender
is definitely correct many similarities, including heavenly "conference" about the earthly problem; angel having to prove that he is an angel; angel asking heaven, "How'm I doin' ?" and getting some sort of response; similar reward for angel at end.
Will accept
Mr. Bevis
.
Please share a little more information as to why you feel
Changing of the Guard
bears strong similarities to
It's A Wonderful Life.
6 Points
awarded (so far !)
2 eps remaining to be guessed,
with strong similarities to
It's A Wonderful Life.
J'ai l'il AMRICAIN !

Mr. Bevis
Cavender is Coming
Changing of the guard

Never say never