I could have written your post. I have IST and have had Supraventricular Tachycardia as well.
I'm alot older than you and have had this for a long time. I have never been treated for it until recently when going to the doctor and he said my heart couldn't take it for the long haul. He also said I have high blood pressure even though my numbers were never over 140/90.
He started me on metoprolol 25mg first and I felt awful and only tried it for a week. He then switched me to a calcium channel blocker, cardizem 180mg upping it to 300mg, and that didn't work, my heart rate was always high still. Now, I am on Nadolol 40mg, and I feel awful, even though it is the first med that has brought my heart rate down. But, it went from 160 bpm to 60 bpm and I feel so bad! I'm glad my heart rate is lower, but I hate how it affects me! I hate any and all medications, that is why I lived with my Inappropriate Sinus Tachycardia for so long, I was used to it, and believe me, I am not used to the way I feel now. I have been on Nadolol for a couple months now. Maybe it will work for you.
I know how you feel, and hope my story helped. I am trying to find out if there is any way we can just live with our IST. I have, but it is a very debilitating condition, and the meds to treat it are just as debilitating for me personally.
I can relate. I'm so sorry you are going through this rough patch. I am 34 with two young children and going through this mess while trying to keep up with them and seem like "happy mama" for them is so hard.
I'm not sure if my advice will help, but here goes nothing. I take metoprolol once per day. The full pill is 50mg!! No Way!!! I think I'd be comatose!!! My cardiologist told me to take 25 mg (1/2 pill) per day to alleviate PVC symptoms. Well, I tried, but I was dreaming weird dreams, falling asleep at work, and generall felt "drunk." This was supposed to "pass" but after 3 weeks it hadn't and I couldn't stand it anymore. I cut the pill in 1/2 again (now 12.5 mg) and take that at bedtime. Once I did that I had NO PROBLEMS.
I have chronically low bp as well. And I suffer from vasovagal syncope at times. The beta blocker was lowering my HR and BP to rates that were just intolerably low. This means that taking 25 mg was making me pass out more often then I have in 10 years. The doctor said that 1/4 pill couldn't be doing anything for me, but I've learned over the years that many of us who have these sorts of "issues" are very sensitive to medications. A little goes a long way.
For me it boiled down to finding the medication that seemed to work the best at releiving my symptoms then adjusting the dose to the lowest possible amount that seemed to do the trick. Even though the doctor didn't feel that the dose I was using was sufficient...it worked for ME.
I realize that your situation is different and that you've tried a lot of options. I just wanted you to know that you are not alone in your frustration. Perhaps others will come to your rescue soon.
Good luck,