As a SO of someone with BPD I would like to know how you cope with the waves of idealization and devaluation? Especially, the devaluation part, the leave me alone part, I don't trust you, ...etc.?

My SO (maybe Ex) has behaviors of BPD as he cycles in and out of idealization and devaluation with me. Devaluation when he is in times of crisis. I sense he fears I will abandon him, because he has problems. And thus at times pushes me away. And then comes back again. 

I already got accustomed to it in the hopes that it would lessen his ups and downs with me, when he sees that I don't abandon him, even when he is at his lowest point. Somehow, I fear one of these times of devaluation might be the last, and he won't come back anymore and this thought doesn't hurt as much anymore, it doesn't make me panic (I had my problems but I am starting therapy now).

However, I would like for that to not happen anyways because I do love all parts of him. And I think we can cope with this at the end somehow.
 
My question is, is there anything to counter the period of devalution? Anything I can do/say or shouldn't do/say? Is there an endpoint? 
Bottomline is I don't want to lose him, even if I don't feel as panicked anymore about it as the first times it (the devaluation) happened. 

