You need to GRIEVE, GRIEVE, GRIEVE. Oftentimes, when we are still going through the grieving process we can get confused and think we don't want to be here anymore. Looking back on times I've felt like that, I'm so glad I didn't try to end it all. I was just deeply upset and missing the person I lost. Everything seemed futile. Instead of being so concerned with what the spiritual consequences are if you do commit suicide think of what the consequences have been for you regarding the way you feel about your life. Get a hold of your thoughts and realize these emotions for what they might be - a natural part of the grieving process. If you don't feel comfortable seeing a therapist, right now, buy books written by people who have experienced the loss you have. Losing a loved one to alzheimer's messes with your reality. (I bet their are books written about people grieving alzheimer-related deaths specifically as this loss, since you were looking after her - might bear its own unique way of grieving.)
A question for you, if you could have the life you envisioned right now or once envisioned, what would it look like? Even if none of these things are curently in your life start right there and keep your head up. For what it's worth, I don't know you, but I don't accept you not wanting to be here.