I have this problem as well. I have fairly long fingers and when bending my fingers (if they are spread apart) my middle and ring come together. Is almost like if they were longer they end up making an x. Im dong lessons here and I do lessons once a week at a local guitar shop. My teacher said that its a lack of muscle thing and with time, stretching, and practice it will get easier. I know my posture is usually good and I use a strap. Ive been doing some of the warm ups for the first 10 minutes of practice and doing a decent amount of stretching and its already gotten a little easier. Check on here or youtube and you can find some good videos on stretching. Its a bit frustrating, but dont give up! Keep going and you will nail it.

peace,

RiffNoob02