I developed SVT about 3 1/2 years ago. In that time I've had 2 urgent visits to the hospital ER for cardioversion (the electric 'paddles'). I had no prior history of it, but since it started, I've had anywhere from 2-3 to upwards of 20+ SVTs per day. All but the two which required a visit to the ER have either stopped on their own, or I was able to stop them with a Valsalva maneuver. They last anywhere from a few seconds to a few minutes.
I tried atenolol (a different beta blocker), but it had little effect. Beta blockers don't really suppress SVTs; all they can do is slow the heart rate so that if a SVT develops, it hopefully won't be as serious. There are specific antiarrhythmic drugs available (flecainide and encainide), but they can have significant side effects and can actually worsen arrhythmias in many patients.
FWIW, I found that regular bicycling seemed to have reduced the frequency of my SVTs.
Given the very high frequency of my SVTs, earlier this year I had catheter ablation. After that, the frequency has been reduced to a few a week, but the jury is still out; it takes a few months for the heart to remodel after ablating the aberrant 'circuits'. Many people need a second ablation procedure to completely eliminate them. Depending on how often you experience them, you might want to talk with a cardiac electrophysiologist about whether ablation might be an option for you. The two electrophysiologists I saw both felt that a frequency of more than about once a month was reason to consider ablation.
Good luck!