ANSWER - Dreams can be pretty powerful things and I'd be foolish to even suggest that just because a dream feels extraordinarily real it cannot simply be a product of the unconscious mind. Having said that though a vivid dream can on occasion be more than a dream. A dream can be a a memory stored in the subconscious that cannot be accessed by the conscious mind but which comes out when the conscious mind is at rest.
For those who subscribe to the idea of reincarnation a dream can be a memory that relates not to this life but to a life previously lived. The difficulty lies in determining whether a dream is a dormant memory of a previous life or a result of stimuli picked up by the sub conscious (you know the sort of thing - you walk past a shop selling wellington boots and spend the night dreaming you're in a rainstorm). A lot of what we dream about doesn't register with our conscious minds - we can't trace it back to an actual experience or trigger point - and so it is quite natural to wonder if the source of the dream lies in a memory from a previous life.
Both the frequency and the intensity of your dream would support the idea that you may be remembering a previous life. The one thing that doesn't is the nature of your dream. Although witches have used broomsticks for diverse purposes over the years, the one thing they have never used them for is the actual physical act of flying. So it is impossible that you could be reliving a memory of flying on a broom as that quite simply could never have happened.
You could I suppose be blending a regressed memory and a stereotype from your current life (the link between witch and broom) and in doing so recalling a past life using modern imagery. I'd certainly say that is possible. However it is equally possible that your sleeping mind is just playing with the stimuli picked up from the (no doubt) hundreds of books and films that you have seen during your life. With something like witchcraft which is both a) common in popular culture and b) to a certain extent aspirational it is hard to draw the line between where recollection ends and subconscious fantasy begins.
I'll also add a further note of complexity - dreams can not only be memories of the past they can also be a foretelling of the future. Your dream could be an indication not that you have been a witch in a previous life but that you will be in this one (or even the next). Again it is unlikely the broomstick is an indication of actual flight (although with technology progressing the way it is I wouldn't rule it out) but it could be a powerful symbol representing the path you are going to take.