While it is certainly normal for you to grieve your son (and the many other losses you have experienced), it is not normal or healthy for you to be experiencing such intense grief that you are thinking of suicide. When grief is prolonged and so profound that it triggers thoughts of self-harm, it needs to be treated more like a depression- and this almost always includes professional help. I would strongly encourage you to find a good psychologist who you can work with on a regular basis; weekly psychotherapy can provide not only a good outlet for your thoughts and feelings, but can also be a place where you learn different coping strategies for managing your grief so that it is not so overwhelming.
I am also wondering if you have ever considered joining a support group (either in person or online) for mother's of children who suicide. Coping with the suicide of a loved one often elicits a very particular sort of grief that is different from the grief induced by other deaths. Having the support of others who have been through something similar might be very helpful for you.
In general during times of grief and depression it becomes extra important to find ways to care for yourself. This might mean making healthier eating choices, exercising more, making sure you get enough sleep, making sure you have enough social support and time out with friends/family, journaling, etc.
With respect to your medication, there are many medications available to treat depression. If the Sertraline doesn't feel helpful there is no reason that you need to stay on it. I would recommend that you have a conversation with your prescribing psychiatrist about what alternatives are available to you.