They must be doing a lot of repetition if it's 21 games. Are they doing things like the second quest for the original Zelda, Master Quest Ocarina of Time, the colorized version of Link's Awakening, the 3DS version of Ocarina of Time, etc? There's just Zelda, Zelda II, Link to the Past, Ocarina, Majora's Mask, Wind Waker, Twilight Princess, Skyward Sword, Link's Awakening, the two Oracle games, the two Four Swords games, Minish Cap, Link's Crossbow Training, and the two DS releases for actual unique releases. I count 17 Zelda adventures and that's including Skyward Sword and Link's Crossbow Training (A game I'm not sure everyone would count as an actual Zelda game).
Link to the Past was a fairly easy game at the time (Although there are certainly difficult sections and a decent amount of deaths, unlike the no death expectation people have with Zelda games produced under Ejii Aonuma).
You're just not used to that style of gameplay and such. If we could take you back in time to 1991 where you had just came off the 8 bit era of gaming and were regularly playing Genesis and SuperNes games, I don't think you'd think anything like you do at the moment.
It's a great game, so keep at it. One easy suggestion is to make sure you have as many of the 4 bottles as you can at this point in your save, and stock up on potions before entering a dungeon. You essentially can get 5 sets of hearts and magic to use on any dungeon once you get all 4 bottles. And don't be in such a rush. Do some exploration so you can do things like upgrade some of your equipment, expand the number of hearts you have, and even expand your magic meter (I think that last one was a sidequest, at least, but maybe you have to do it to progress in the main quest).
Keep at it and I bet you'll feel it's much more reasonable with its difficulty level than you're currently thinking by the time you finish it. And you should be more than ready to experience some other 2D Zelda's. I'd personally suggest Minish Cap for the GBA. It's probably the last great 2D Zelda we'll ever get and it has some beautiful 2D graphics, traditional gameplay (With some concepts from the 3D games implimented as well), and some interesting new ideas (This game apparantly is where much of the inspiration for Skyward Sword came from, according to some article I read on IGN a while back).
In light of your difficulties with Link to the Past, I doubt you'd ever be able to get into the original Zelda, Adventures of Link, Link's Awakening (Although that one is probably the easiest candidate to really get into among these games I just listed), or the two Oracle games. But if you end up actually being glad you played Link to the Past by the time you're done, I bet you'd enjoy Minish Cap.