x = [1, 2, 3, 1, 2, 1, 2, 3]
idx = []
for i, (num1, num2) in enumerate(zip(x, x[1:])):
    if num2 < num1:
        idx.append(i+1)
for i in reversed(idx):
    x.insert(i, 0)      # or, x[i:i] = [0], which would be overkill in this case