If you are saying that your instructor insists on using the drysuit for bouyancy control, it is time to move on to another instructor (and likely agency because, if it is the agency I think it is, that is why your instructor is suggesting to use the drysuit to control bouyancy--stupid!). Keep practicing in a pool until you can take two to three minutes to ascend from a depth of ten feet to the surface in a smoothe, consistent pace.
Yes, the amount of time I said is right. You drift up just a bit, vent, drift up, vent and so on throughout the ascent. Just a small bit of movement is all it takes to require a vent.
How do you have your exhaust valve set? It should be all the way open so that, once you start to drift up, you only need to raise your arm (and possibly tilt a bit heads up--just a bit though) for the gas to vent automatically. Did you instructor tell you to turn it up a few notches? Wrong!
Next:breathing control. When you breathe in, you start to rise (this is why you learn this by doing the fin pivot). Exhale and you start to sink. Right? The trick is to learn to time your breathing (still in a natural way though) so that you start to breath in as you sink and start to exhale as you rise. That way, you have much less swing between how far down and up you will go with each breath. I know this seems impossible now. Just try it; you will get the hang after a while.
Now, you are going to put it all together. Make sure your suit is vented. Get neutral. Time your breaths so that there is little swing up and down. You will need to do more venting as you ascend at the end of your dive but you will still be neutral during the slow ascent that you already did in confined water. Once you can do these exercises in a pool or other shallow confined water, you can start to work in open water and deeper depths.