So you willingly accepted that someone take your power and now you want it back? I won't even ask why this happened. However, hear me out.
Your power is a part of your spirit...its integrated into your soul and the essence that is you! How can that be removed from you? The answer is, it cannot. However it can appear to be gone. Just as a severely depressed individual feels that their power is limited or gone all together, when you remove the drive, determination and ability to focus, you strip a witch of their ability to cast.
This is extremely easy to fix. It isn't that you have to seek out your power and return it to yourself. It's still there inside of you. What you have is basically damaged energy. Energy that has been made to slow down or go stagnant. I'm sure since then, you've been suffering from the emotional and mental effects that this caused. Never ever allow someone to do this. It is harder to reverse if you were willing because you accepted the change.
There are many ways to go about fixing this. Mostly healing excercises. Since I can't see you in person, I have no idea where this has effected you. Usually it's around the solar plexus and mind, but it is a domino effect. Like a physical body, energy circulates, and when one area becomes ill or damaged, it triggers a chain reaction of darkened energy over time. A reiki master would be excellent for this, but if you wish to try yourself, there are a number of energy healing visualizations, meditations and rituals to choose from. Don't stop working on it until you feel like yourself again.