Sometimes for me I am not sure if it is the med having less of a positive impact or it is just the time is worse pain wise. I go through periods where I have flares that are worse then others and it is not from anything I did. Well I don't think anyway. I think it is great you feel a positive mental impact though. Really when I was feeling better mentally it was helping me cope with my pain and health problems better.
I just read back and you stated you are sensitive to the other meds like neurontin. Like I said when I first got dx I started this med and had a really hard time. Then about 2 years later I tried it again but very small dose and increased very slow. With each increase I felt very bad but it did lessen as time passed and then felt ok. I know some side effects are too bad to stick with the med but some do pass.