I'm so sorry to hear about all that. :(

First of all, it is definitely not your fault that she decided to commit suicide. I don't know her reasons, but it is never just one thing that makes you say "okay, yeah, time to die." I'm certain she was depressed and thinking about it before it happened and, like Silenia said, even if your boyfriend HAD been there at that moment it probably wouldn't have prevented the situation from eventually occurring.

I'm not very good at condoling people after a death happens, and being lucky enough to never experience a death of someone close to my myself, I can't tell you exactly the right course of action. But your boyfriend definitely seems to want you there, given that he called you before he even called the police. I think you're being hard on yourself; if you were a bad girlfriend or taking advantage of him, why would he be with you at all? Be there for him when he needs you, and that's really all you can do.