Hi TK68:
I thought I'd reply from a different perspective. I'm a new diver as of last year (I've only made 14open water dives on my one trip last summer), but had the same bouyancy problem with my contained water experiences in class. I don't know how tall you are (I'm 5'4"), but you and I are about the same weight.
I had a hard time in the pool staying at the bottom w/o a lot of weight. I felt like I was rolling around like a turtle on its back, bobbing back up to the surface. Rest assured though that it's easier in open water!
You'll have a lot more space to maneuver (the pool was too crowded for me to properly execute the skills - I did ok) AND it's easier the deeper you go because of the increased pressure. In my first dives I found the first 20 feet the hardest to manage, but once I went deeper than that I was much more skilled at controlling everything, including bouyancy. Since most pools are only about 12 feet deep, it can be discouraging when you don't have control of your body.
When you do your open water dives (if you haven't yet), pay attention that you don't have a similar problem with your safety stops at 15 feet. I'm hippy (sadly, and fat floats!) and w/ a 3mm jane became more and more bouyant as I got closer to the surface. So, at my 3 minute safety stops I had to make sure that my bcd was free of air and even on occasion had to hang on to my boyfriend's hand so I wouldn't drift upwards.
I truly had the same issue when I was learning and thusly the same concern about going into the open water. I was surprised and pleased at how much easier it was in the depths and salt water. And I too, breathe deeply and slowly (yoga breathing) and was able to manage my bouyancy on my dives with breath control.
I hope this gives you some assurance and good luck!