Each time you were changing the tensions, were you using the same person on the same machine to string it, or did you change tensions with a different person and different machine? Keeping with one person and telling him/her what you are experiencing can make a big difference.
Might be a mental thing too. It happens to anyone. I got some prestrung racquets from TW, and mentally I thought they wouldn't be as good as the racquets I strung. I hated them at first. I didn't want to waste it and just cut them out, so I played with them until they broke. As I got used to them, I liked it.
If you feel like you are spending too much money, you may consider stringing on your own.