So. I'm totally ADDICTED to Heartstrings right now, and am looking for more like it :)

Even though I find them to be nothing alike, I think you would enjoy "Sassy Go Go" and "Weightlifting Fairy Kim Bok Joo"! They're just as cute and I enjoyed them... a lot! :)
I LOOOOOVED WLF Kim Bok Joo.  It was such a nice heartwarming drama.  I'll have to check out Sassy Go Go next.  THANK YOU!!
Do you watch Chinese dramas too? The main couple in Across the Ocean to See You was super cute!
I actually have never watched a C-drama.I'm still relatively new to kdrama (as in started watching in March and haven't taken a single day break yet! LOL), but one of my friends on tumblr recommended me one about swimming thats currently airing. (My Mermaid Man????)
Thank you!! I'll check out Across the Ocean to See You!!!
You probably are looking for light coming of age dramas like Heart string and weight lifting fair KBJ. So i wanna recommend:

1. Love O2O (chinese drama but ooohhh so romantic and fun. Super highky recommend this drama like weightlifting fairy KBJ
2. Monstar (high school drama with so much fun side)
3. Reply 1994 (a goos roller coaster drama that you may love to eternity or hate to eternity, depending on who you ship with the female lead)
4. Shut up flower boy band (set in highschool. There is romance, there is friendship and there is music like heartstrings)
5. orange days (jaoanese drama, a little less romance and more of the college life)
6. Age of youth (not really centered on romance but over all great coming of age drama)
My Mr Mermaid! I haven't finished it yet, but it is amazingly cute.  Also, if you enjoyed Heartstrings, you should enjoy You're Beautiful. The two leads from Heartstrings are also main characters in this other drama. 
Another thing that I wholeheartedly recommend is Fight My Way.
I have seen Age of Youth and LOVED it. I'm nervous about the second season of it though because the cast has been shaken up a bit, but I'm going to give it a try.  I Have also seen You're Beautiful and that's how I was recommended heartstrings.  I had SLS so bad in You're Beautiful, but I was also extremely in love with the chemistry between Mi Nam and Tae-Kyung.

I will be adding every single one of these dramas to my list of ones to watch.
You guys are amazing.
Drama friends are the best ones to have :)
Guys I've completed Heartstrings and am moving on. I believe I'm going to try Love o2o or shut up flower boy band next. Please keep the recs coming. This community is amazing :)

Secret-Had me all tore up. Uncontrollable Fond-Most hated it, had me crying. 

 Faith:

Secret-Had me all tore up. Uncontrollable Fond-Most hated it, had me crying. 

Secret is on my list of dramas to watch in tandem with a friend (if we ever get to it! lol) but I cannot wait to see it.  

I also can't wait to watch Uncontrollably Fond. I think it looks great and Kim Woo Bin is one of my favorite actors in dramaland.