You appear to me to be a very concientious fish-keeper. And that is good and hence I'd say to you that your personality based on that is very well suited to fish keeping so don't 'quit' as some would have it.
It is clear that luck has not been on your side, and I would say that one makes their own luck, but that does not appear to be the case with you as you have done everything right: The generic advise on this forum is sound & is fantastic for the beginner.
It is also clear to me that your LFS that you got your stocking from may be having problems of their own & I think you've inhereted their problems when you purchased fish from them. I wonder if going back into their shop you'd see many of their tanks in quarantine?

If so, perhaps a level-headed letter of complaint may not go amiss.
Installing a UV filter of some kind I think is good for any tank and will help guard against future infections. Also investing in a Quarantine tank may also be a good idea, however I do not practice what I preach here: I popped into my LFS today to fish-gaze and a lot of their fish appear to be getting white spot!... & guess who bought some fish from them last week!
It's worth to know that a tank cannot suddenly get ick unless it's been introduced somehow, in other words, YOU have not 'caused' your fish to get diseased & hence you are being unfair on yourself to assume blame for this - it must've come from your LFS.
Good luck curing your fish & don't get too down-hearted. Let's face it, this hobby for you is only going to get better!
Andy
PS... I'd leave keeping plants for now, get used to keeping fish first.