Step 1) Get your feet checked out. I'm not convinced that an upright bike is going to solve this problem (in fact, it may be worse because your feet are well below your heart). Also, an upright _may_ cause additional problems (sore butt, sore wrists, sore upper back, etc.) that you have not yet experienced.

Step 2) Closely examine your pedaling technique with your 'bent bike. Are you mashing the pedals or spinning (i.e. not a lot of down force on the pedals). Higher RPM will cause less pressure on the feet. Going clipless or adding clips to your pedals may help with higher RPM pedaling. Bonus: since you have a trike you are not going to fall down :-).

Step 3) Once you have gone through the prior 2 steps then consider looking at upright bikes.

Do NOT get a fixed bike right now. Save that until you are very comfortable riding an upright bike and can go up the hills in a medium gear.