I’m so sorry to hear that you’ve lost both your husband and your son - that is so much for one person to face. It is sad to hear that your son’s death was possibly suicide or an accidental overdose, and that you are suffering from guilt over not checking on him. Guilt can be a very common part of grief, but it sounds as though your son kept his problems hidden from you.
I hope that it helps even a tiny bit to be able to share things here. The people on this site have all lost loved ones and will understand some of what you are going through. While you wait for more replies to your post, you may wish to read other posts in the Losing a Partner and Losing a Child categories to see what other people’s experiences have been.
For example, in this conversation, you can see posts by @KarenM and @gail1, who have both lost sons to suicide: I lost my son by suicide. And in this conversation, you can find @Jackie012008, who lost one of her sons to a drug overdose: Lost Both sons. Please feel free to post a reply in any conversation if you see someone you would like to talk to.
As well as posting here, there are some other organisations that can offer further support:
Survivors of Bereavement by Suicide: https://uksobs.org/
Bereaved by Alcohol and Drugs: https://www.beadproject.org.uk/