To be honest, I'm not all that sure I agree with some of what Tom says above. I've never bought the idea that beginners have to start on a narrow tip opening mouthpiece - I certainly didn't when I started on tenor as a callow youth 32 years ago. OK, I would say that a 0.80 mouthpiece is a slightly larger tip opning than would normally be recommended for a beginner, but I just don't accept that it's of a dimension only fit for pros and advanced students. There are so many more factors than tip opening alone in determining how easy a mouthpiece is to play.
What I'd suggest you do is set a target time, say two or three weeks time, and play the sax with your new mouthpiece for at least 20 minutes a day between now and then. Make sure you use a pretty soft reed. If you're still struggling at that time, order a B3 from t'internet. You can get them for about 18 quid. However, I think you'll adapt OK to the B5. The most important thing is just to play the sax. As much as you can. Enjoy it, sax playing's great.
Thanks for your input i do love the tone im now getting from my sax and the second practice was much better,im defo keeping this mouthpiece but think i will try a few others too,maybe a yamaha 4c and esprit.