Don't thank me yet Marie. I'm not quite done with you.
OK, if you can easily equalize on the plane, then it seems apparent that you don't have any sinus restrictions and/or small eustachean tubes. Therefore, we need to work on your technique when diving.
Now then. I want you to equalize, right now, sitting in your chair. Just like you would on the plane. Move your bottom jaw forward just a bit and/or swallow? Feel the slight pop like you get on the plane? So now you know what you should be doing a time or 3 before you get into the water.
Myself, I start equalizing on shore. I do a couple of those and then again on the boat just before descent. Then while still at the surface but in the water, I do a valsalva. (pinch nose gently, gently puff into ears) I've found frequent little puffs work ever so much better than a few big honks. You must equalize early and often. By early, I mean you must equalize before you feel any pressure and most certainly, before you feel any pain. By often, I mean after every inhale.
If you are still feeling fullness in your ears after diving, it would be my guess that you are equalizing too aggressively. I suspect that you've been having trouble for so long now that you've managed to psych yourself out on this issue.
There is nothing wrong with descending slowly, as long as you are equalizing early and often. Going back up when you've gone too far is a good plan for your ears, that's true. However, going up and down like that will surely screw them up. And I don't mean to say don't go back up, you must if you are having problems. You just need to find a way to get past going up and down like that. Remember what it was like in the pool? And when you do your pool refreshers before going back out on another dive trip? Those few feet can really screw me up with the going up and down.
So, you need to find a way to get equalized BEFORE you descend to a point that requires you to ascend to equalize.
OK, so moving right along. Now you are equalizing softly, gently on shore, on the boat and at the surface immediately before descent. Next, you've begun your descent and so have exhaled all of your air. You've dropped a foot or two. Equalize gently anyway, you'll still have enough air left to give a little puff, even so. Now you will inhale just a little bit (because you don't want to fill your lungs so much that you ascend a bit when you are still trying to get down a few feet), that quick little inhale is enough to equalize again as you exhale. Inhale, equalize as you exhale, inhale, equalize as you exhale, continue until you are at depth and have met up with the group. Hold your depth. How are your ears? Looking good.