Yeah, some meads don't turn out the way we hoped, that doesn't make them bad. Failure is probably too strong a word. Creatively different flavors... yeah, that's it.
Here's what you should have done different: NOT A DAMN THING.
You've done everything right in the proper proportions. If it is bubbling away, then let it ride. Write down the date when you pitched the yeast, then six to eight weeks from that date, (Mark it on your calendar.) take a reading with your new hydrometer. Gallon jugs are useful in that you can simply drop them in and take a reading without having to pour a sample.
Presuming the bubble fermenting has stopped after the six to eight weeks, two conditions have to be met: The reading must be 1.0 or lower, and two: the fruit should have fallen to the bottom of the jug. I've noticed that if the fruit in the primary has fallen, then the flavor is much better.
Raspberries have a high sugar content, so be prepared for the yeast to make a comeback when you get around to racking it. One of my pineapple meads blew it's cork yesterday. (The dog lapped it up and got drunk while I was at work, the sneaky bastard.) Despite filtering, racking and waiting a few extra weeks while it cleared, some yeast cells were dormant. The process of racking stirred and woke them up. They decided to munch on any remaining sugars and I now have naturally carbonated pineapple ginger mead.
You may want sparkling raspberry mead. Typing that out makes it sound like something you should shoot for. I am assuming you know what Camden tablets are and what they're for. My mistake is I always underestimate the amount needed to kill any yeast cells. I've a few batches that turn out sparkling. That in itself is not a failure, but a feature. The wife says my traditional tastes like champagne, but perhaps she's biased.