Difficulty with Posture

I am noticing some pattern in my recent practice. I find that mental exertion, as directing attention to the breath, is quite easy for me; yet, when it comes to devoting an amount of time to formal practice, or even sitting formally and upright, I have trouble. For example, I will tend to meditate in a slouched or even lying position. **EDIT**: There is no pain, or specific problem when I meditate upright, just a sensation of general discomfort, or an unpleasantness. But, I feel maybe it is more of an aversion towards sitting upright than an actual physical problem. My question is: **How can this resistance best be explained? What is the best way to counter it?** Thank you.