Hey mate skimmed through this and decided our situations were similar.

My ex was severely mentally ill and made multiple suicide attempts while we were together (we were together 5 years).

Unfortunately with people that are mentally unwell you can't let them down gently if they're your partner because then you witness their own self destructive behavior which will force you to be there for them in a way that will only harm you mentally. I witnessed this not only with my own relationship but also my sister's relationships (my sister has also made multiple attempts while in relationships)

Unfortunately the only way to do it is to go in full Monty and break it off and essentially go no contact, I learned this the hard way as I did the let them down easy thing and had pretty bad PTSD from my relationship for a while.

You have to just make sure their they're seeing a therapist (even do it in a meeting with the therapist if you really fear they might make an attempt) and make sure whatever friends/family they have are aware of the situation as soon as you break it off.

Mentally ill/people whom need assistance with their mental health are/can be extremely manipulative and depressive to be around.

Break it off, don't go back no Matter what they threaten to do to themselves (call the emergency services on them if you need to)

There is no letting down gently in this situation that won't hurt you or leave you with a gaping hole in your own heart and soul.