There's a difference between inexperience and stupidity. For you, the former is a fact, the latter is not obvious. Many new divers "pack air" in the lungs, meaning that they don't exhale fully and so are overly buoyant. Instructors will often overweight all new students to get around this problem and get folks underwater. It's very unlikely that you need more than 2-3 kilos at most in fresh water without a wetsuit. But the real issue here is that you need more time in the water. With those repetitions you will get more comfortable and work out the weight and trim issues. Have patience and don't beat yourself up.
