This one should not be so difficult owing that I practically gave it away. Let's review the issue:
It appears that Moshe Rabbenu takes a time-out in the middle of his lengthy tirade of Kellalos to give us a commercial break - a message from the Sponsor about what we have been doing wrong. This commercial break seems to comprise three pasukim - Devarim 28:45, 46, and 47. In other words, pasuk 28:47 - that we have not served G-d with joy and good-heartedness - is a follow up of the 2 previous pasukim. I.e., it is part of the problem. Then, from pasuk 28:48 and on, we return to our regular programming and continue with the [final] solution.
In other words, pasuk 28:47 has very much to do with the 2 pasukim before it and very little to do with the pasukim that come after it.
This is how I understood the pasuk for over 3 decades. I asked my 2 study partners (both of them are older than I am), and that is how they understood the pasuk as well. For decades. This seems to be the simple way to see the pasuk and I think virtually everybody sees it that way.
I think it's a mistake.
I looked at the pasuk this year and saw somehing I never noticed before. Let's look at this pasuk again:
מז תחת אשר לא עבדת את יהוה אלהיך בשמחה ובטוב לבב מרב כל
Now lets look at the pasuk after it:
מח ועבדת את איביך אשר ישלחנו יהוה בך ברעב ובצמא ובעירם ובחסר כל
Did you notice something? I think pasuk 28:47 has a lot more to do with the next pasuk - and a lot less to do with the previous pasukim - than we have thought all these years.
Pasuk 28:47 is not telling us why we are enduring these kellalos! That concept was wrapped up in the previous 2 pasukim - primarily pasuk 28:45 which says that we have not been observing the mitzvos.
Pasuk 28:47 is the beginning of the next round of Kellalos! It is the opening premise of pasuk 28:48 and it is saying this:
Your next Kellala is that in place of serving Hashem your G-d in an environment of joy and good-heartedness out of sheer abundance of all...
Or, in other words -
Instead of sitting around a Shabbos table in fancy clothes with 9 kids and 25 grandchildren, fressing on Chulent and kugel with your pantries overflowing with groceries and your wine-cellars stocked floor to ceiling and all your bills paid and singing zemiros and saying over vertlach, etc., etc., etc., (pasuk 28:47)...
...That you could have and should have done but you didn't; instead of all that...
You will instead be serving your enemies (and their gods) in foreign lands in an environment of hunger, and thirst, and nakedness out of a lack of all (pasuk 28:48)....
Now we see that this pasuk is not giving a new and perplexing reason for the Kellalos. It is a Kellala! And now we understand the conjoining "vav" at the beginning of pasuk 28:48.
I truly think that this is the rudimentary pshat in the pasuk.
תכלה שנה וקללותיה, תחל שנה וברכותיה