How does one serve G-d with simcha when one is aware of flaws, sins, and other bad habits that should cause one to also have bitterness.
Not asking about depression and other negative emotions, rather, how can one have Simcha - but also have bitterness over wrong doing?
Background:
Hashem created you. You are to him like an only son.
If he made you, it's because he needed you to be around.
You are not a mistake.
That being said:
Hashem obviously knew what would "come out" of you (though, as the Rambam says - at least I think that's who said it - : "It is (almost-?-) impossible to understand how free choice and Hashgocha Protis fit together").
Explanation:
You should be happy because you are Hashem's son, and nothing and nobody can take that away from you.
You should have Merirus (bitterness) about your flaws, in a constructive way, when making a Cheshbon Nefesh (not 24/7).
It would definetely be a Stirah, were you meant to have Atzvus (depression - a naynay, always!), or even (the sometimes needed) Merirus (KN"L), at the same time. But you are not.
You should be Tomid Besimcha, and only have specific times dedicated to true Self-evaluations, which may (and hopefully will) bring to constructive bitterness, meaning to want to be better.
Edit:
You may want to look at Tanya, chapters 26-28: Chabad.org/[...]chapter-26.
Note: If you are ever in doubt of whether your temporary bitterness (as depression has no right time) is From your Yetzer Tov, or the opposite (R"L), ask yourself:
"what do I want to do because of this bitterness?"
If the answer is "get closer to Hashem", great!
Now leave it alone and do that!
But if the answer is anything negative, such as "give up, I'm bad, I stand no chance anyways, it's too late", then tell that bitterness: "out of my way!"
The fact that although you are holding where you are, your service is still allowed and even demanded, is alone a good cause for celebration.
In reality we should be told that learning Gemara, Davenning, Succah, Shabbos etc. is not for you. Just be a citizen of the world in which you spend most of your time. And yet, our service is allowed, requested, required and appreciated.
This is what I have in mind (when I remember to) when I say in the Brachah, אשר קדשנו במצוותיו וציונו, that He sanctified us with His commandments and He commanded us to...
Its hard to find a source, it time gone by this was not an existing phenomenon! people "slipped" did teshuvah and carried on a straight life! nowadays that so many of us are addicted to sin we have this problem. If you are looking for a credible and original source see sefer Tanya chapter 29 and chapter 31 who directly addresses your issue
The simcha is the realization that even though we now understand the depth of what we have gone through and the potential problems that we have caused ourselves, Hashem is now saying, "Come and be with me and understand that I have forgiven you."
It is only now after Yom Kippur that we can actually feel the simchah of the gmar din (whether for good or better) that has been signed on Yom Kippur and will be sealed as a result of ivdu es hHashem Besimcha. After all the tochcha says that we would be punished because
You did not serve Hashem besimcha
The Hebrew word simchah shares some of those senses. But it goes deeper. The Sefer HaCarmel (Malbim) differentiates between gilah and simchah. "Simchah indicates an inner heartfelt joy that is constant." Citing the verse (Psalms 31:8) "I will exult (agilah) and rejoice (esmechah) in Your kindness ..." the Malbim explains that gilah connotes a sudden burst of delight. Then he relates it to a verse in Isaiah (25:11), "This is G0D; for Whom we hoped; let us exult and be joyful with His salvation."
The Alter Vorker Rebbe (d. 1848) explained this with a story: A king came through a tiny village with his royal procession. One young boy was so unnerved by the spectacle that he threw a rock at the king's carriage.
Immediately, soldiers rushed to catch the boy and were about to kill him. But the king shouted, "Don't harm him. Take him with us back to the palace."
In the palace, the king arranged the finest teachers for the boy. After 10 years, the king sent for the boy - now a young man. As he entered the throne room, the young man bowed low to the king, then stood in reverential silence.
"Do you remember," asked the king, "the day we first met, when you threw a rock at my carriage?"
The young man was seized with shame and fainted.
After Yom Kippur, the Vorker explained, we are like that young boy. We spend 10 days coming closer to the Divine, until we reach the pinnacle of Ne'ilah. Then, at Maariv, we suddenly realize, "Oy! What have we been doing all year!?" And we pound our hearts in shame and ask for forgiveness.
So where do we get the joy?
Rabbi Shlomo Carlebach (d. 1994) said
Like the boy who suddenly realized in shame, "What have I done?!" - after Yom Kippur, we can be left feeling we have no place in this world anymore.
But the Divine says, "Come; come with Me. I have a place for you, a place to shelter and protect you."
That place is called the Sukkah.
