Left-handed trumpets are very rare. Learning on one as a lefty is probably about as common(and practical) as learning to drive a stick(standard transmission automobile) in a British car because doing the gears would be easier with your left hand. I'm a lefty too and right-handed playing has never bothered me. And my right hand isn't nearly as dextrous as my right hand! But trumpet playing isn't like writing, drawing, cutting, etc. It's not particularly intricate, it's just up and down. I'm sure you'll get used to it soon enough.