Feel as I am stopping hims

Good afternoon, this is my first  time writing. I am my husbands caregiver,he has colon cancer which has metisized to his lungs and liver, doctor said he will be on chemo for the rest of his life. He retired last year at 64 yrs old, he has always been a busy person so this has taken a toll on him going to chemo every other week. We have been married for 37 years, he was married before no children, we have 3 children. Here's my issue one day I was out he was down so he went on facebook checking it out and up came his ex wife so he sent her a message., they were in contact with each other thru his gmail account which I did not know. She wanted to meet with him, so he finally told me, I thought I was ok with it until his phone rang I picked up & I saw he had sent her a music video my man-Trace. Atkins it upset me deeply, he knew I was upset I asked to see her message he said he delected them but there were new pictures of her. To say the least we argued he did not understand why I am upset, he did not see her he told her I was upset about it she said her husband was ok with it?? Really...well she sent him a wool hat said she had something for me but did not send it, no surprise there. She then sent me a card asking me women to women to understand that I need to keep her in the loop, which I did not reply. My husband told her that he was not going to message any more because of me so 3 months went by & I found out he message her again up dating her on his last treatment I was angry first he lied he said that if she or him contacted each other he would tell me well he did not, finally he showed me her reply on his message and then he said her to tell her he would not message anymore as I was upset as he lied she replied saying she was pissed no he needed to man up and stand up to me so they could continue messaging and it's a Shem he has to hide his messages/lie.

i just do not understand why he needs to be in contact with her as she cheated on him more than once and hurt him, he said it was curisoty but it Lead to more. She said in a message that she was sorry for hurting him and she still loved him. She is married has 2 grown kids &  is about to retire. 

Please be honest with me am I being selfish for not letting him see her? Every time I go out and come home I don't trust him, I need to get over this. I know cancer changes people-angry, depression we are not physical any more not even holding hands he says his body aches 

please help me I am at wits end