this is supposed to be a high side only and that is why the load is connected between the source and gnd.
You should remove the load and connect the drain of the lower side mosfet in its place.
The optocoupler output transistor can replace the Q1.
The D1 voltage is the same voltage that powers the bridge, it is 15v in your case and assuming that your mosfet has a max Vgs of 20v there will be no problem with 15v.
Q5 is just a pre driver of the totem pole , if you want to use a different design then you can remove it.
Alex