I'm really sad to read this. I hope you will carry on and keep playing healer!
Of course, messing up as a healer is far more detrimental than as a DPS (or at least, it is immediately visible). Yet I think Orbonne could be basically healed by one healer in each team, really; meaning that there is not so much relying on you. It's too easy to blame it on the healers when people die for any reason. And mind that this was your first run. I can assure you people did not shine on release day either.
You don't bear the full responsability of your party wipes : you have you own mistakes, but you also have your team's mistakes, you have a co-heal that should power up the healing if they see their partner in trouble and such.
There's only practice that will help you getting better. Maybe practice on 8man raids (although they are arguably harder to heal than 24 man raids). Play safe, at your own pace. Find your strengths and your weaknesses, and balance your gameplay on this. You must learn the fights to know what damge is expected and which tool in your kit is the most useful for it.
And it's a game. Relax a bit, don't put too much pressure on yourself and end up not enjoying playing it, it's not worth it at all