The fact that you understand, the reason you are losing velocity is because you have no separation in your back hip to back shoulder before foot strike, is a big bonus for you.
There are many drills to increase separation in your delivery. The trick is finding the right one for you.
I would start with your back leg. When your back knee bends, when your hips are moving towards the plate, does your knee bend over your back toes? If so this is forcing your hips to open early. You want your glute or butt to kick out towards the plate to start your delivery. This will prevent your back knee from bending over your back toes and your hips opening early. Remember “Separation” occurs when generating good momentum in your lift leg and following that with opening your hips a split second before front foot strike; while pinching your Scapula to hold your shoulders back.
Tim Lincecum is a great example of this
I have an article you should read after working on your “Separation”. Best of luck!