Do you want her support to maybe try to recover deep down? Is it because you want to warn her so that it's less of a shock for her? Or, is it more that it's such a worrying thing to be contemplating and you want her support without her trying to stop you doing it?
If you want her help to possibly try to recover, you could tell a half truth. Say you are experiencing suicidal thoughts which, you don't have plans to act on but, you're struggling to live with them, maybe.
If you're hoping people will support you with your decision as well as supporting you emotionally on the run up, I think that is quite a lot to hope for. It maybe depends on their ideas around the right to die. How they perceive your problems and life. I imagine they have to accept that there's really no other good alternative. I think a lot of people struggle to do that though.
Plus also, the legal implications I suppose. No one wants to get in trouble for assisting a suicide or be quized as to having knowledge but not acting on it eg. from angry family members looking for someone to blame.
Sadly, I think it's a tricky position to put someone in. I suppose that's why somewhere like here is so valuable. Maybe not the same as real life but, we can support one another annonymously.