What can I do to loosen up my frozen shoulder that hurts when I do certain weight training excercises?

OMM and/or PT. Osteopathic manipulation and physical therapy can both be very helpful in treating frozen shoulder. This can be a very frustrating problem but, with proper treatment, you can get your shoulder working again.
PT. If you have a frozen shoulder you should really work with a physical therapist. He/she will give you a good set of exercises to get your range of motion back if possible.