If you are looking to lose weight then yeah definitely its not ideal to have my shake in its full glory. If you were to halve or third it it could be good as a meal replacement for the morning when you don't have time to properly prepare something substantial to eat. Also, maybe drop to water only and limit the peanut butter a bit more if u want to shed calories.
Also, as bungee says, get training. the more muscles you have, the faster you will burn calories and in turn shed the fat. This is one of the reasons why the massive guys are able to cut like 50lbs from off-season to competition standard (although many other factors apply).
by the way, MRP means meal replacement powder, and is an effective way to boost up your protein levels that you require for building muscle, as well as giving you a healthy meal when you have no real time to prepare, at a relatively low cost.
good luck and keep posting up your progress. Perhaps a good time to start a thread in the progress journals part of the site at:
http://musclechat.co.uk/viewforum.php?f=23
Nick