Sorry to hear of your fiance's struggles. My wife had to deal with this with me. I was put on testosterone cream and DHEA tablets. Wow, what a difference.
One simple thing to do first. Get her hormones checked. You want to direct her to a doctor who is a specialist in bio-identical hormones. There is a link for a referral.
https://www.womensinternational.com/..._referral.html
The PCS brain gets hormones all messed up. She may also be lacking in brain nutrition. Search this forum for nutrition or supplements. She likely needs additional B-6, B-12, folic acid etc. There are blood tests for folate and B-12. Her hormone doctor can request these tests.
Be patient. Flat emotional and sexual response is as frustrating for her as it is for you. She likely just does not know how to understand and discuss her flat emotions.
And, read Dr Glen Johnson's TBI Survival Guide at
www.tbiguide.com You can download it as a MSWord document and print it out to highlight sections.
Be patient. She is still in there. She wants to get back to normal too. Be prepared that you both may have adjust to a new normal.
My best to you both.