Overstressing the aircraft? Hardly likely. The certification limit is at least 3.8g, and a 45 degree bank doesn't even get to 2g (someone do the math please?). 3.8g requires serious pulling (on the stick/yoke) and serious puffing (breathing).
What are you using as your bank angle reference? Are you looking at the horizon or at the artificial horizon? What I tend to do is roll the aircraft with my eyes on the artificial horizon to get to 45 degrees exactly, then look outside for the angle between the real horizon and the cowling at some point. Keep it there, pull back on the yoke and put a little extra power on. It might not be the correct technique (your instructors advice is paramount anyway) but it works for me.
If you tend to roll out early, it might be some latent fear. Together with your instructor, do a few 60 degrees banked turns. It doesn't matter if they go perfect, but they might help you overcome that fear and 45 degrees will seem easy after that. Better yet, get an aerobatics plane + instructor and do some 75 degree turns, rolls and loops. 45 degree banked turns are dead easy after that.