It's actually quite a common to have leaves around that cover other leaves, but I see what you mean.
That is a repeating motif, so that is more aesthetic than functional(much like a Celtic knot). The problem is not really your motif. Mandalas have radial symmetry. Yours has an incomplete one, regardless of the motif you used. It lacks the radial symmetry needed for it to be functional in terms of meditative practices. Nice drawing, though. Your center is radial; however, the symmetry is not preserved when you make it spherical, in your drawing, at the outer edges. The outer edges of the petals don't have a radial symmetry to the center. Versus adding more petals to make it look 3-D, you should shade them. Along the inside, or outside, of the lines you should make them darker and then use a slightly lighter shade around that darkened line. It will make it look 3-D without the extra petals spoiling the symmetry. Shading creates a 3-D effect without having to add a lot of elements in that depth is implied via the presence of varying levels of darkness and contrast. So versus having a lot of elements in a complex arrangement where that can make preserving symmetry hard, you can make use of shading techniques to suggest depth with less elements that are distracting. The amount of elements you have makes it distracting, which is why I said it is not very functional or practical. For example, say you draw a circle. If we assume that the light is shining from overhead, making the bottom lines of the circle darker while shading the inside via curving strokes done to the bottom from lighter to darker(which would be your darkest part) would make it appear to be spherical with one element. We can apply this same concept to your sketch where we subtract distracting elements and still imply to the eye that it is 3-D. It seems as if you attempted to make it 3-D via implying there are other shapes behind it via having their edges stick out, but that spoils the symmetry. The grid paper you used actually makes it easy to see in that starting from a point and moving diagonally from the edge of a petal to the center, the line would not hit the center. It hits the bottom portion.