+1 and as you fall, it is ok if you fall into a flat trim position, it is what many people do.Flood your suit at the surface. It may be too baggy in spots and too tight around your chest/abdomen causing an air bubble. In my semi-dry I often would have to wait for air to migrate out or break the seal at my face and let it out faster to descend easily.
Cross your ankles to prevent inadvertent kicking up and hold your inflator above your head while venting until your hand is under. If these steps don't work then it needs to be addressed 1 on 1 with an instructor.
If you fall “front up” (upside down) it means you were leaning backwards too much.
I never had air trapped in a wetsuit but maybe try to flood it before to descend like @Boarderguy said.
If you swim down, don’t do it more than to overcome the first few meters, unless you know you are correctly weighted: I saw someone swim down all the way to 20m and who was unable to stay there and went back to the surface as he was underweighted …