I have been a runner for nearly 20 years now. In high school, I suffered the occasional IT (ilio-tibial) band injury where it got so tight that even if I tried to run through the pain my speed was severely restricted. And I had a coach which encouraged running through pain :( As time has gone on, such injuries have become more and more frequent and require at least a month of down time to heal. Even with rest, it seems these days my body really is not recovering from it and my IT band is having a hard time loosening up at all. It seems leg-length difference (5/8") has played a big role in this condition. 2 years ago I got a new pair of 1/2" custom orthodics at a sports doctor I was seeing, but those do not keep me completely injury free. Am I doomed for a lifetime of problems or is there something else that can be done to regain that springiness and flexibility in my IT band?
I know a lot of people who have had some really severe and longlasting IT band issues and have done the work and then successfully come back to running.
For severe issues, which yours appear to be, I would suggest getting good advice from the appropriate specialist. In this case, I would find a good physiotherapist with good knowledge of running. I would expect them to put you on a program of rest, following by a lot of stretching.
The issue is that you will have a blister on your IT band where it crosses the tightest point, right beside your knee. You need to get that blister to heel properly, then get your IT band to remain loose while slowly building up the volume.
If you do this too fast then you will re-blister the IT band and have to start again.
I cannot speak to the leg length or orthotics issue. Personally, I remain unconvinced of the effectiveness of orthotics. Your experience may vary.
Iliotibial band syndrome (ITBS) management and prognosis is something you should discuss with your health care provider. They can help you with your specific situation.
There is surprisingly little high quality information about ITBS in the literature about prognosis.
Information from an ITBS systematic review
Here is information from a systematic review1 about ITBS.
studies confirm the benefits for the conservative treatment of ITBS in runners; pain medication/injection, stretching of the iliotibial band, hip abduction exercises/pelvic drops to strengthen the gluteus muscles and advice about training, inlays and shoes. Unfortunately, to date, no (randomized) clinical trials have investigated the benefit of these different modalities in isolation. Although iliotibial band bursectomy and arthroscopic resection of the lateral synovial recess proved effective in runners with chronic (>6 months) ITBS, the studies investigating these techniques were small.
conservative treatment appears to be beneficial in the management of ITBS in runners, although the evidence supporting this comes from studies with small, heterogeneous samples. Surgical approaches appear to be effective, and the arthroscopic technique would seem especially appropriate because it allows assessment and treatment of any intra-articular pathology. In the future, it would be interesting to compare these treatments in an RCT with more participants.
Information selected from UpToDate
Even UpToDate2 had nothing on prognosis, and this is what they stated on treatment:
There is little high quality evidence to inform the treatment of iliotibial band syndrome
TLDR: There is little high-quality scientific information about the treatment of ITBS, and even less about prognosis. Your question is best aimed towards a health care provided experience in ITBS.
 https://www.uptodate.com/contents/iliotibial-band-syndrome - accessed 16-04-2022