I have been off work (this is the fourth or fifth time, but the longest) for two and a half years now due to my chronic pain issues. I applied for the "first category" of my LTD, which is "disabled - own occupation" and was denied; all it took was a letter from my surgeon, and I ended up approved. That lasted for 25 months, then I had to be reassessed for "disabled - any occupation". Unfortunately, I am one of those people who often "looks SO good!" that people can't believe I'm suffering as I am, so it's been a fight!!
When I had to apply for the new category, I took my time doing the forms. I had the head of ER at our local hospital write a letter explaining the extent of the treatments he's had to do in the ER and his opinion on me returning to work. I had my old boss write a letter explaining how he watched my health deteriorate and distract me more and more at my job, I had my massage, physio and acupuncture therapists write official reports on THEIR findings, and then had about eight friends write letters saying who they are, how long we've known each other, what we USED to do together, and what we STILL do together (not a whole lot, obviously!) and whatever else they chose to share. I attended my Independent Medical Evaluation right after seeing my own rheumatologist,who actually USED the words "disability" and "can not hold down gainful employment" in the same sentence....THANK YOU!! Obviously the IME doc gave me a fair assessment, which is how I felt at the time as well, and I got a letter on Monday saying I was approved.
When I submit my forms, I tell them EVERYTHING. I tell them about the week long or months long setbacks and what I've had to do to "settle them down", I tell them that I've tried swimming again, but it really caused pain here and here, so I had to stop, you name it.
One thing I was advised to do (by a psychologist who was at a residential pain clinic I attended last year) is make SURE your info is accurate and truthful. If they ask you if you can drive, NEVER say you can't if you can...even if you can only drive a block. Make it clear that you can "drive for a short distance, but at great expense to me", or "can drive comfortably for fifteen minutes, but neck and back pain increases substantially", etc.
I could go on and on here, but my point is document, document, document ANY changes in your symptoms, abilities, treatments, drugs, etc., and use it to your advantage when you're filing for benefits. Tell them what you've had to give up because of this (i.e. I made it clear that I've not only been unable to work, but I no longer bike, ski, do long hikes, socialize as much, craft, volunteer, organize community events, etc.).
Be your own advocate, and all the best in your "journey"!