I'm late to this thread, and you probably won't understand this post - because it's very late and I am very tired lol - but here goes anyway.
Sounds to me like you're the kind of person who gives a thing your all... like you focus intensely on the subject in hand. A bit like direct healing, instead of distant healing - you know? I tried healing and it drained me to hell, then I realised I was stroking the cat or focusing on the bird and WILLING it well. Yes the cat and bird got better, but I was left almost cross-eyed from the effort - and I believe I could have done it just as well using spirit healing beamed through me, not direct healing beamed from me. Does this make sense? lol
So if you could try working on degrees of intensity or layers or levels of it... pull back your energy a wee bit and practise that way, like switching up & down gears in your mind. Work on feeling that shift, feeling that level of intensity. I'm sorry I can't be clearer on this but I do know what you're saying and I have done it myself and got it sorted now. Is just hard to put into words!
Now this really is useful and I know exactly what you mean.

Maybe if I hadn't experienced it, it would be harder to understand.
The distinction you make between willing [any "patient" or client] well - healing power coming
through instead of
from the healer - that's exactly it. And yes, I give things my all, as you say.
I will use this way of seeing the problem as (I hope) a way into allowing the reading to come
through instead of
from me. A metaphor, a mental image, really works for me when it comes to making changes - this image (not exactly visual but I hope you know what I mean, now) feels as if it will be useful. I have previously tried to read the cards giving less of myself, and haven't managed to. But this framework will help, I think. I'll make a point of noticing the level of tuning-in as it's happening.
You got it sorted, in you own case - do you think that simply doing more was part of the "cure"? I mean, might I find that when I have enough experience under my belt it will just click into place and become something I can handle? - a bit like a child suddenly finding a knife and fork aren't impossible to control after all, just a case of enough hours' familiarity, plus maybe a bit more maturity (or muscle strength and co-ordination in the case of cutlery!).