Thoughts, thoughts...at times, my worst enemy

I'm embarking on a new journey; a new direction for my life. As I face these new challenges with hours upon hours of studying, I find myself wavering....and thinking "is this the right path for me?" Of course I want my choices to be in line with God's Will for my life, I always have. And though, I have been the type of person that always seeks His Will for me, I honestly don't really know if I have ever been on the path He truly wants me to be on. Well, then come the thoughts, and those really make things a bit more frustrating. I started this new journey with joy, because I truly want to teach. I actually really love doing it, and now as I endure the struggle of studying,  thoughts creep in such as, "maybe you are making the wrong choice," and "how are you going to pass this," or "what if you can't pass this."  And even more discouraging are when the regrets start with thoughts such as, "you shouldn't have left your career."  Little after little, those are the thoughts that can trip you up and keep you from moving forward. I have discovered, that those thoughts can really be my worst enemy. 
When I reflect on the past me, and the person I used to be with past decisions I made, I see those enemy thoughts were right there too. Unfortunately I have always been a person that tends to lean towards anxiety and worry when facing hardships/decisions/stressful events. I see now that some of that is biological and some of it is unhealthy patterns. Oh and of course not to forget, I'm sure the devil enjoys seeing us fall and make bad decisions that hinder us from being the people God wants us to be.
So I have come to understand that those thoughts don't define me. They are only thoughts. But when I allow them to keep filling my mind and tell myself they are true then they do start defining me. Because then I simply make choices that will either be positive or negative. It is so easy to get off course. I have done it so many times in my past. I realize my thoughts aren't always pleasing and good for me. For me, the only times I truly have peace with the world is when I focus on God and His Word. There are so many unknowns in life that Trusting God has been where I find Peace. I don't say that lightly believe me, I have friend's that would tell you-Trusting God- has been a huge struggle of mine in my faith walk. I am not all the way there yet, but I have grown so much in the past year. 
At the end of the day what does worry or regrets really do for you? I can tell you for me, all it ever does is hold me back from moving forward. And usually if I listen to those enemy thoughts, one or two or a combo of the following occur: stress, discouragement, negativity, anxiety, and depression. Thinking about what you are thinking about is important. Some good scripture to ponder on:
Philipians 4:8 Finally brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable- if anything is excellent or praiseworthy- think about such things.