Also had it for 15 years, I was sharper in my 20s than now but now and I think age and MS have affected that. I realise I'm at my best when not stressed, well rested and happy. However I'm not always in control of those. Stress is easier to deal with as I talk to my employers about my condition and I avoid anything stressful when possible. I still feel useless some days and have bad brain fog and can't think clearly, it gives me a lot of imposter syndrome.
Im currently moving somewhere else to get better rest. And my happiness is in progress, I tried anti depressants but I get anxiety and I tried CBT but it didn't work either. I do also swim regularly which helps too but still not enough. Im hoping this move will help. However its gone on for so many years that I think I will seek a professional soon.
Covid and the pandemic definitely affected me, I couldn't walk properly for weeks after recovering from it. I used to do 20min walk every day to recover and just to keep active and get out. My depression got worse (was getting CBT in days before lockdown) but a few friends of mine play regularly online so I got to be social at least and talk about it. To add to this, I also quit my job before lockdown to travel so I was unemployed for a year in pandemic. I remember going to cafes when things partially opened to practise interview questions and did little projects. I finally got a job the next year.
I'm still worried I won't be strong enough to travel one day, my max distance walking is 1.5km, give or take, without a rest, I could go a lot further only a few years ago.
I know MS affects everyone differently and it can be a very hidden condition. My life could of been quite different without it but it could have been worse. Im lucky to have found a job coding and the silver lining of the pandemic is wfh. I see it like you have to play with the cards your dealt and whatever you can change for the better then try to. It's not been an easy road till now but I try and remain hopeful for the future.
I know it must be even harder considering your circumstances and your health. I hope this helps to know your not alone at least.