The songs you find easiest to 'pick up' probably have similar chord progressions/structures. The chords and scales your may currently be practising,are training your ear whether you know it or not.
When you learn a new song try practising it in different keys to further familiarise your ear. If you have difficulty in transposing/changing keys, check some sites that will give you this information.
All the best and don't let frustration lead to failure