I won't bore you with the details. I cried myself to sleep. I have no friends, and summer is coming up. In other words, i'll be all alone for three months with only the internet to entertain me.
I would love a good happy shoujo manga ( or even just a comedy). Just a super sweet one, no love triangle, no annoying girl who gets in the way. I love a dense heroine like koi dano, or last game. I need to be cheered up, thanks in advance xx

Beauty is the Beast
Beauty Pop
Akagami no Shirayukihime
Horimiya

Here are some of my favs that fit this criteria. Feel free to take a look.

Nulla in mundo pax sincera
"Always go too far, because that's where you'll find the truth." - Albert Camus

Tomo-chan wa Onnanoko!

Two of my favourite cheer-up mangas:

Gals!
Juuni Hizoku no Palette

They're both a bit older, not sure if it's your style, but I like them a lot. I own the print English copies and always read them when I'm in a bad mood.

You could try Ore Monogatari!! - it's pretty funny.
Dengeki Daisy
SA

Um, I had never realized this before, but usually I don't like the ones with dense protagonists.

However, I remember some that fit your criteria:
Shinigamihime no Saikon
Furou Kyoudai (the horror tag is just there because of ghosts, but really, it's more of a comedy)
Don't Cry, Girl is also quite funny, but the heroine isn't dense and it's just 1 volume.

It might be a good idea to browse some oneshots. You can usually get a feel for an author's style this way. Plus, they are often very sweet stories. ^^
Though you might want to stay away from the tragedy tag/genre right now, from what you say.

Since you said it could just be a comedy, I feel I should mention Beelzebub. It's shounen, but always good for a laugh.

Yotsubato! is by far the cutest most cheerful thing,

P x P is a short but fluffy story.

I always need these type of manga to cheer me up.

Highly Recommend:
Gakuen Babysitters - No other manga has made me feel so warm and fuzzy.

Others:
ReRe Hello - Happy, warm romance
Aozora Yell - more happy, warm romance
Otomen - Silly, light romance
Ore-sama Teacher - Silly, dense comedic characters
Nodame Cantabile - same as above but more slice-of-life
^ recommend the anime as well
Hachimitsu to Clover - feels like Nodame
^ recommend the anime as well
Love So Life - Fuzzy, warm family life
Chihaya Furu - shoujo good feels + sportsy "must persevere"
Kono Oto Tomare! - same as above
Hana to Akuma - actually has a bit of drama, but Hana makes everything cute.
Gin no Saji - focus on farming/everyday life makes it feel fulfilling
Barakamon - Comedy and slice-of-life
Kimi to Boku. - Comedy and daily life
Nijiiro Days (MIZUNO Minami) - Comedy, bromance, daily life

Outside of this:
Sports manga always cheer me up. The positive attitude and team spirit is infectious.
So maybe something like Days or Haikyu!!

I would need to look at my full manga list to come up with a really good list of multi-volume romances...but since that's not the case I'll give you the comedies that pop into my head immediately:

Yotsubato (mentioned before but would like to emphasize) very cute and funny
Azumanga Daioh (by the same mangaka as the above - 4 volumes)
It's Not My Fault I'm Not Popular (can be depressing but it makes me laugh too)

Here are some of my favorite oneshot manga that I really enjoyed reading during past summers. Discovering these was like finding little treasures ^^

My favorites:
Happy Every Day
Haikei, Nanashi-sama
Daydream World
First Love (Aruko)

Other awesome ones:

Houkago Love Age - Story 3 - Kirai Kirai Mo?
Hidarite no Love Letter - Story 1
Ai no Tame ni - Story 1
Hatsukoi Hakusho - whole collection
Aoi Futari - Story 1
Hi Ga Kuretemo Aruiteru - Story 2
Koi Ni Ichiru - Story 1
Koiiro Omoi - Story 2
Boku no Sekai Kimi no Real - Story 1
Warui Ko no Mikata - Story 3 - Citrus Sweet
Oikakete Mo Chikazuite Mo - Story 2
SOS - Story 2 (waterworks warning!)
Class 2-G's Romeo and Juliet! - one story
Orange Kiss - the whole collection but especially Story 1 Orange Kiss
Sumire wa Blue - one volume, I would skip the side story at the end
Forever Honey - warning, waterworks are probable for this one!!
I Want - one chapter
3 Years - one volume
Nanaco Robin - one volume
Koi wa Tenka no Mawarimono - Story called Ikenai Candy Love
Suki Kirai Suki by Obata Yuki - one story
Kiss, Zekkou, Kiss - one volume
Apple Cheek Love - story called Ringo Hoppe no Koi
Kimi no Kachi - the whole collection
The Kidnapping of Minja Jo?s Boyfriend - one volume
Deep Love Virus - one volume
Line - one volume
Always Together - one chapter
Mimi wo Sumaseba - one volume
Romanticist Egoist - one chapter
Nonchan to Watashi - one chapter
Dear Green - one chapter
Shuukatsu!! - Kimi ni Naitei - one volume
Spicy Pink - one volume
Saikou no Kiss wo Ageru - Story 1 - Heartbeat Mail (gyahahaha!)
Benkyou no Jikan - Story 3 - Green Days
Lemon no Himegoto - Story 1
Cherries Fight - Story 1
Anti-Chocolate - Story 4
Try Me Boy! - Story 1
Hizamazuite Ai Wo Chikae - Story 1 and 2
Halfway - whole collection
Aquarium - whole collection
Taiyou wa Kimi ni Kagaya ku - one chapter
Prince of the Student Discipline Room - one chapter
Tonari no Danshi - one chapter
Atashi Hitori de Daijoubu - one chapter
By Hero - one story 🙂
Bye Bye Little - Story 1
Celeste Blue - one story
He Has A Girlfriend - one story
Joyful Days? - one story

I love oneshots because you don't have to invest a lot of time in them. If the story is well done you get the "happy giddy high" and if it isn't you can quickly move onto the next.

I hope you enjoy some of these during your sad time and that they can keep you company for a least a while 🙂

I love to discuss one shot manga so if you want to talk about any of these after you read them I would be happy to!