stop saying to yourself I CAN't DO THAT,...playing by ear is all about you interpreting the music in your own way.....just memorizing isn't really enough..you have to play the song effortlessly sometimes different every time because you can playing by ear that's the beauty of it vs the written chart if your playing alone, everyone cannot do it, but those who do enjoy the benefits of walking alone without your eyes glues to a page all the time,.....it's a different story when playing with others and you all have to be in sync playing the right parts etc,...

lets face it there is NO magic pill for playing by ear...sit down every day WITHOUT THE MUSIC in front of you, and play your songs, over and over again, only repetition which might take years so what? where you going is the key to this?.... you will get the right results and you will be a better player because of it,...don't give up, make your mistakes, forgetfulness, over and over, every day, ....but eventually you will start to be able to just sit down and play songs, sing, with your mind not a chart,....music is a beautiful journey when it's played & created thru your soul,....go for it and good luck....
don't be lazy try it!