The slight roll to your finger that others have mentioned sounds like pretty good advice to me. I'd also try playing songs that you are already comfortable with in a barred position. For example, playing a classic I-IV-V progression (like G, C, & D or A, D, & E) at the 3rd and 5th fret. There is much less resistance at that part of the neck, and it pulls your hand in closer which can straighten our the muscles in your arm a bit if you have a smaller frame; this makes for less stress for your muscles across your list and the base of your thumb. So, while you develop the technique with less factors fighting against you, you can focus just on that technique, and not on learning a new song, or really feeling like you are having to work on something completely new - just different - which can make for a better mindset as well.