While my mother never had access to a gun I spent most of my childhood and adolescence keeping an eye on her. Every 3-5 years she would attempt to off herself by trying to overdose on the psychiatric medications she was put on or by doing something more cringe worthy like trying to drink pinesol and lemonade mixed together. While she didn't live with my father and I very often, she lives close enough that he still, to this day, checks up on her, etc.
A lot of this happened when I was really young or a teen. I shut into myself at a young age from having to visit a parent at the hospital numerous times after having her stomach pumped. What you're going through is going to hurt, in one way or another.
#1, do not blame yourself. I blamed myself for a lot of things for so many years because it felt like it was the only thing I could do when no one else was giving me any direction on how to feel and what to do about it. I turned into an extremely anti-social person who was easily depressed and struggled with many parts of day-to-day life because I weighed myself down constantly.
#2, from what I understand, your mother had some form of an illness that may have not been treated correctly. She may not have been getting all the help she needed for it and may not have realized it herself. It isn't your fault that she, in a way, succumbed to an illness. It's not a point-to-point comparison but try not to focus on the fact that she was the thing that finally ended her life.
People who have heart diseases that continue to eat terribly and never exercise despite "doctor's orders" are the major contributing factors in their deaths. If someone has cancer and will not/cannot seek treatment, they are gambling with their health. As sad as suicide is, it still hits home much harder because there is so many emotional and social factors that are blamed for it occuring. It makes it easier to treat it like something that's more preventable than heart disease and other "genetic mishaps" that cause physical illnesses. People like to think self-inflicted injury from mental illness can be willed away when in reality it is just as much an uncontrollable threat to a person's well-being when untreated as any other physical illness.
#3, give yourself permission to feel crappy, cry, be hugged, take some time off work if you can manage, seek out a counsellor/grief counsellor (and find a different one if the first person you speak to isn't someone you feel comfortable with). Expect people in your family and close friends to be at a loss too. Offer support if you can or point them in the direction of support if you're overwhelmed. Don't try to be the hard, strong man.
#4, don't forget to treat yourself to simple pleasures in life when you're struggling with this. A really good cup of coffee, your favourite meal, buy a nice plant for a bright room in the house that cleans the air (peace lily's are great for this), don't hermit in the house- go for a walk in the daylight and if you have a gym membership, EXERCISE. Even if it's just running for 20 minutes it is proven to make people feel better and will give you something else to focus on. Clear your head, if you will. Don't neglect hobbies, watch your favourite shows, etc. Don't sink into a hole too much, or at least try to be aware if you are and ASK FOR HELP from someone you trust if you are. It feels much better just knowing someone else knows your pain.
Remember, life goes on. Things will feel awful at first but time changes things. Keep yourself busy and don't obsess over the details of events, what you did or did not do. And don't let family and close friends do the same if you can help it.
(sorry if I'm not very coherent, I'm typing this from a touchscreen phone)