Well, the first part where fixing it begins is at launch. You want to make sure that at launch you stay as close to 90° as possible. This will prevent from going to the poles. Now to circularize your orbits you want to do exactly what you have been doing. Burning until one part is at your desired altitude (Apoapsis normally). Your Apoapsis the highest part of your orbit. Now what you want to do is to circularize by first going to the Apoapsis (AP). Once there, look at your navball on the bottom, and look for a green circle. The one you want has no X in the middle. Then point yourself directly onto it and burn. This is called burning prograde. Burn until your Periapis, lowest point, is pretty close. That should end you up with a nice and straight orbit. Hope I helped, TNCM