Hi Laura and welcome to the forum. As the others have said you’re not on your own for feeling frustrated having considered yourself a bit of a high achiever. I wasn’t aware of the “MS Type person” Pat mentioned but it does seem appropriate to me too. Like you and many others, I have balance issues and was quite unsuccessful at the gym, the worst experience for me was falling off a treadmill. I think they should definitely be avoided.
Like Nina mentioned, a Neuro physio is your best bet. I try to do the core strengthening exercises mine gave to me, I have a gym ball at home and there are so many strengthening exercises you can do on it. The best thing about seeing a specialist physio is that they know how much our nervous system affects our bodies and look at things like our gait and posture and try to make our lives easier and safer with targeted exercises.
Swimming is very good for strength and cardiovascular workout and as the others have said, there are drugs for spacticity. Whammel hit the nail on the head though. High expectations are unrealistic and very demoralizing. You’ll probably never run marathons again. You must set different goals now, your body has changed and unfortunately with this dx you won’t get back to where you were years ago. That doesn’t mean you give up though, you can still enjoy life and achieve success but just in other ways and with new targets.
This forum is a lovely place where everyone understands the frustrations and have experienced symptoms and challenges you’ll never find in a book so I find them very helpful and motivating. I hope you do too. Take care.