I'm not quite clear on what exact BCD you are using, but it looks like you've got more weight than the BCD can compensate for. If that's the case you're into "not safe" territory.
From
AP's manual
View attachment 748005
A 15L steel is going to be another 4 or 5 kilos negative at the start of the dive (I think - someone will surely correct me if I'm off).
My guess is your biggest issue is how you were trained. Instead of looking for more equipment, take
@tbone1004 's advice, or find a new instructor/shop. If you can find someone who trains students in neutral buoyancy during open water, that person will probably be able to help.