Mitzvah Gedolah Liyhos B’simcha Tomid

R.Noson Sternhartz zt"l  writes, that R. Nachman Breslover said ,

          "Mitzvah Gedolah Liyhos B’simcha Tomid."
I don't understand,

 

1)Where do we find this Mitzvah in the Torah or Chazal?
None of the Monei Hamitzvohs count Simcha as a Mitzvah.
Mitzvohs are to be done B’simcha,
but it's not a Mitzvah in itself.

2)  How can you say “Tomid” if there are times when not only
is it not a Mitzvah but an Aveirah ?
In Hl Aveilos # 391 – 1  “Aveil  Assur Liyhos B’simcha “.

3)  Why is Simcha, more of a Mitzvah Gedola than other Mitzvohs
which are not Gedola?

I found only one Mitzvah Gedola in the entire Shulchan Aruch  
“Mitzvah Gedola L’haspid Hameis  Koroui” ( It is a great Mitzvah to eulogize
a Niftar properly).

The Tur (E.H.65) has one more, “Mitzvah Gedola L'sameach Choson v'Kallah”--.
It's interesting The Mechaber in Shulchan Aruch skips the word Gedola
(Mitzva L'sameach---.)

There is another Mitzvah where Gedola is mentioned in Shulchan Aruch,

Hl. Pidyon Sh’vuyim “Ein Lecho Mitzvah Gedola K’pidyon Sh’vuyim.
(There is no greater Mitzvah than Pidyon Shevuyim)