Well, first of all, the talking tips from the sidebar are here for just this.
Mostly though, you need to change your understanding of this. First, you can't "control this." You need to let her move at her pace, and understand that you cannot fix this. It sucks, but it's the truth. Also, try not to use the phrase "it will get better"....it's often true, but the link to rule 4d in the sidebar explains why it hurts more than helps. :/
Most importantly, understand that suicidal thoughts or depression (although not all suicidal people are depressed) do not have anything to do with "having a pretty nice life" or "getting everything you want" or having money, or good parents or "never having to worry about anything."
Being there for her is important, but understand that this might not be something that will better just by being there for her. That isn't because of you....sometimes it just takes more than two people who don't know how to fight it.
If she hasn't, ask her...important word, ask her....if she would consider talking to a counselor or a therapist about how she feels. Maybe a doctor, if she or a therapist feels okay with that. Neither of these solutions is a fix all, but they are a plae to start so that you sort of know that at least they do work or they don't or they sort of do/don't.
If it's been this way for a year and a half, understand that it will not go away quickly. Even when it does leave, these thoughts often return, sometimes worse than before, so you need to either be prepared for that, or exit the relationship soon.
I really think you should check out the talking tips I linked above, and the rest of the sidebar links as well, if you can. :/