Is talking about quitting to your supervisor a point of no return? I have been feeling miserable about my PhD degree situation for a bit of time, and I just started therapy. So far I've tried not to take drastic decisions such as quitting, because I know I am strongly influenced by my condition, but lately I can't think about anything else.

I wonder if it is possible to convert my contract in a research assistance, or what is a graceful way out so to continue deserving recommendation, or what is an adequate resign notice period, etc., but I know it would much better to finish the PhD degree, and so continue trying.

Is talking about quitting possibilities with your supervisor a point of no return?
Talking about quitting with your supervisor (or someone else in your supervisory committee or department) is not a point of no return. In fact, it can be a turning point.

PhD students all go through ups and downs and part of the job of the supervisor, and others in the department and university (head of department, counsellors, etc) is to help you get through the rough patches. 

If you do not talk about it and things get so bad that you actually quit, then it may be a point of no return.

Full disclosure: I quit my PhD. The first my supervisor heard of it was the day I quit. Luckily, someone else gave me another chance a few years later.