... and as for taxying...
If you're flying something with a yolk (eg Piper PA28 or Cessna 150/152, there is a huge temptation to "steer" it - as if it were a steering wheel. On a calm day - and with an instructor if you discuss it first - you could try keeping one hand on the throttle and the other on your lap. It's not good practice normally since you need to use the control surfaces even on the ground, but if "steering's" your problem it might just help.
OTOH, given your age, it may just be that you haven't got the knack of steering yet. I remember when I learned to drive that it took quite a few lessons to stop myself "sawing" round bends. You don't taxy that much if you're learning to fly at a smaller field and I can well believe that it would take a few lessons to get smooth turns going on the ground.
(Oh, and some planes can just be hard to steer!)
You should be taught a number of drills whilst you're learning to fly possibly including "FREDA", "APT", "PAT", "BUMPFICHH", "HASELL" and "HELL" (there are probably more) and variations thereof. Have a look in your Trevor Thom Book 1 or equivalent to find these. As your flying develops, some of them sink in better than others, but by the time you qualify you're likely to find that they're almost instinctive. When you change instructors (for whatever reason) there's usually a discontinuity which leads to some things being done twice and unfortunately a different set of expectations or worse, some things being missed out.