AvatarNathan Richer

echoing Bradley Betts – you should work on thoracic spine and shoulder mobility.  use a peanut or gemini along your spine, moving segment by segment and removing restrictions gently.  

shoulder mobility is also important – there are tons of videos on mobilitywod to do that. if you can i would opt for something harder and more “pinpoint” than a foam roller. i think you will get better results if you can get a yoga tune up alpha ball or supernova and roll with that. smash anterior/medial/posterior delts, triceps, traps, rhomboids, shoulder blade area, lats. 
test/retest the wall angel after every single area smashed. if you see change in ability to do it, note that area for further work. if you don’t, move on to somewhere else.
as for the overextended lumbar spine, there could be many culprits. i would try releasing your hip flexors first as they could be pulling your lumbar spine towards your hips in the front if super tight. so gut smash the psoas, use a roller (battlestar works better than foam roller) on the top of your quads. also use the couch stretch and do that every day for at least 2 min each side.  
the other thing i would try first is, before raising the arms in wall angel, achieve a good neutral pelvis position, and then brace STRONGLY in the lower abdomen, and then try to raise the arms but do not give up the pelvis position for greater height. if you can brace strongly and get your arms all the way up, then it is a motor control issue as well. all of it works as a system – mobility, motor control.
good luck and report back your progress!