"Keep trying" is not a solution or a fix for a short luteal phase - that's an acceptable suggestion for unexplained infertility with a typical regular cycle!
Progesterone is a good starting point if you know you're ovulating but a short luteal phase means your progrsterone level is dropping too soon for a fertilised egg to implant. It simply just artificially keeps it raised so you're on a level playing field.
The NHS don't want to know though - GPs don't have enough of an understanding so you get packed off with the typical "keep trying" nonsense.
Go private - get an overdraft, a loan, a credit card whatever it takes! It's not that much money and will short cut all the stress of the NHS bullshit!