I don't want to put words in anyone's mouth but I think maybe Tridus, myself and another DPS friend could take you through a few things to at least try to give you some input on what you're doing in terms of WHM healing. Tridus and I both main WHM and while we aren't super hardcore by any means I think we could give you some insight about dungeon healing for sure. If Tridus tanks, you heal and me and our friend DPS I think we could certainly try to evaluate where you're having trouble and give advice based on that.
You've got me on discord, let me know if you're up for it. I'll try to organize the rest for one night in the coming week or so.

I honestly think with as much effort and genuine desire to do well as you're putting into this there is no way we can't find some ways to help you out. This isn't pity or anything, all I see is a healer who needs to be able to take the training wheels off in a safe enviroment and I think we can do that as a group. Confidence is a big part of healing (especially healing while doing DPS) and you can't get that together until you have some positive runs under your belt.

Let me know. Hope I can help!