I am so incredibly sorry for your loss. I cannot begin to imagine your pain.
Please do not fall into the pit of self-blame. You may have let him come home on Tuesday and he may have slipped out anyway to be with her or the whole scene could have played out 2 weeks later. Nothing you could do could ultimately change the outcome.
I am so glad you had the opportunity to love and love him before he went. You can carry that in your heart forever. Hugs and hugs and hugs.