I've been playing for the past 4 years, and I agree being a tank or a healer is hard, especially when people start blaming you for everything. But, over time, I learned that I shouldn't care about what they think or say, if you did something wrong, they should've been able to tell you what mistakes you did, and try to help you.
On the other hand, I met some nice people that helped me when something like that happened, and I got better at healing / tanking. I started Healing/Tanking in Cataclysm, back in 4.1.
That time, I joined a Portuguese guild, and since I was a complete noob at what I was doing (I was leveling as Healer, until level 60, where my GM approached me and asked why was I taking so long to level up), I used to talk alot on Guild Chat, And I said that I was leveling as a healer, and he told me to either level up as tank or Dps, because it's faster... Anyway, I only was leveling as Healer because I thought I would level to 85 all through dungeons... I was wrong, spent more time questing than doing dungeons :P
Anyway, the time spent by, and I finnaly reached 85! Yay... And I asked my guild what they needed the most... As I was expecting for them to say, they said to play whatever I like the most, that they wouldn't force me to go to a role I didn't like. Well, I started gearing up my Tank spec more, than my Healing spec, because at that time, I got to RDF groups way quicker tanking. Through all that time, sometimes my older brother saw me tanking and gave me some tips too (Taunt that mob, use a CD, etc), and that helped me alot to this. As soon as I reached 340 Item-Level, it was raiding day, and my guild was going to raid Bastion of Twilight (I applied for all raids I could), and the GM again, talked to me if I could tank Halfus. Well, needless to say, I was nervous and said I was really low item-level, but he took me anyway. We did fine, the trash killed me a few times, but we proceeded, there I was, looking at that giant... Until they explained the tactics to me. Every hit I got from him, took me like 15% of my HP, and yes, I was using Word of Glory plus CD's at the same time. We managed to kill it at the 3rd try, because someone kept dying at that fight. Anyway, that kept me going until now, and managed to succeed until then, and I'm sure, with time and focus, you will get there to! :)