First of all do not blame yourself for his mother's death. Those 3 hours wouldn't have made a difference, for all you know she was waiting to do it and didn't want anyone around the house when she actually kills herself.
Right now the only thing you can do for your boyfriend is show your support, be there when you can. I'm sure he'll understand that you can't be with him all the time due to your exams/whatever it is coming up.
Just keep in mind that you have been with your boyfriend for over a year and if you were the deciding factor in it, she probably would have killed herself sooner then that. You never know what was going on in her head to do it.
I'm sorry for your's and your boyfriend's loss.