Since Wicca is only around 50 years old, it is still a religion that most people find later in life, rather than having been raised Wiccan. As such, a great many new Wiccans are faced with the sometimes difficult task of leaving one set of beliefs behind while embracing a new set.
Unfortunately, many people expect this to be as simple as changing a pair of shoes. And when it's not, they start to question their new-found faith.
While I have no facts to offer, I would like to express some of my own thoughts and opinions. If nothing else, I want to let you know you are not alone.
As I just stated, switching religions isn't like flipping a light switch. It can be downright difficult, and the extent of that difficulty has no bearing whatsoever on whether you are making the right decision to switch. Just because your mind may continue to linger on beliefs of your first faith, you shouldn't interpret that as some sort of sign that you have made a bad choice.
It can take work and conscious effort to change one's patterns of thinking. When I gave up Christianity and embraced a Pagan path, it took months of intentional thought to change my perception of Deity from male to female. I can't even remember how long it took me to shake the fear of Hell. My heart had made the switch but my brain took longer to catch up.
Another aspect of switching troubles, is that some folk choose to blend their old faith and new one rather that switch completely. Now, fundamentally there is nothing wrong with creating a new path from different teachings. Nothing at all.
But I strongly recommend that you think about this. Could it be that you are simply unable to let go of some old ideas, and want to keep them around like a spiritual safety blanket? Mixing two paths that have conflicting teachings can lead to further confusion for you later on down the road. So think about your reasons and really consider your choices.
So, how do you make the switch easier? Well, there are many factors to consider, namely what religion you are coming from and how deeply immersed you were in that religion to begin with. My only suggestion would be to take time to learn and study as much as you can about Wicca or whatever your new path is. One reason your mind may reach back to old ways is because you are unsure of the correct answer within your new ways. Learn and keep your mind focused. One day you'll realize that all of your old thoughts have left the building.