Was your treatment all chemo, radiation, or a combo and did you have surgery as well? Chemo wrecks havoc with your entire body;so depending on how long ago your treatments stopped, it's little wonder your erections are weak. Good news is you're 18, and a very good sign that prioritizing your erections and penis health is important to you. Btw, AM/PM wood means morning/evening erections (non-induced by masturbation or PORN).
A few more questions, though...
Have you had successful intercourse prior to your cancer? Were there any issues with your erections or performance when masturbating or having sex before your cancer? How's your overall health now (weight to height ratio, diet, exercise plan, etc.)?