There are a few points I’d make here.
First, disease modifying drugs (DMDs) are licensed for CIS where the neurologist considers the person is likely to develop MS. So while many neurologists aren’t offering them, others are. In your position, I would be asking for DMDs. The reason is that even though you’re not diagnosed with MS, the chances are you could be. Obviously with your second MRI, you may find out that you are in fact now diagnosed with MS. So you will get a DMD in that case regardless.
The point to this is that future relapses will (assuming you have them) increase your likelihood of disability.
The second point is that you can get a drug specifically to act on fatigue. If you can persuade your neurologist when you see him/her, then ask for Modafinil. It is strictly speaking licensed only for narcolepsy, but neurologists did prescribe it off label for MS fatigue and it worked very well. Just lately it’s been harder to get a new prescription for it, but it’s worth asking. Explain how your fatigue is affecting you, particularly how it’s preventing you from working.
Failing that, there is another drug, Amantadine. It is reasonably easy to get this prescribed. It’s not as good as Modafinil, but it does work.
If you find that you can shake off some of the mental fatigue, you may be able to concentrate on getting yourself more physically able too.
Lastly, there are other drug therapies that may work better for nerve pain, some that perhaps will help you better than Gabapentin does at present. You could ask about Pregabalin. Have a look at https://www.mstrust.org.uk/a-z/pain Towards the end there is some information related to drugs which are used for nerve pain. Many of the drugs used for nerve pain have the potential to increase fatigue. This includes Gabapentin. It also includes Pregabalin, but as I’m sure you realise, we all react in differing ways to the various drugs. So swapping one drug for another might help. It’s worth a try anyway.
Lastly, do you have any physiotherapy help? Perhaps there is some available which might help you to get back into a better physical state, maybe not where you were, but just feeling better in yourself.
I think that 6 months into a diagnosis of CIS, even if it is converted to a diagnosis of relapsing remitting MS is too soon to be saying that your days of physical work are over. It may be that Scudger is right and you’ll need to think about a change of career, but don’t write yourself off too easily. Try to make some other changes, just as you are with diet and vitamin supplements. (May people take Vitamin B12 too which is supposed to help with fatigue.)
Best of luck.