It might be that your negotiation ends up in a stalemate, because there are incompatible interests.
Of course, you don't give up easily. You want to be persistent, but also flexible. You want to stick to your guns for as long as you can, and at the same time maintain a good relationship.
And you want to use all your creativity to reach an agreement that is acceptable for the both of you. Read more about this in my blog ‘Positional or principled negotiation?’.
But still... suppose that there is an issue that you can't agree on.
Realise that one of the results of a negotiation could be 'no deal'.
You could estimate how your negotiation partner will react to this, by imagining what their BATNA is. BATNA is an acronym for 'Best Alternative To a Negotiated Agreement'. In other words: what happens to your negotiation partner if you can't come to an agreement? What is his or her alternative? If there is a second source supplier that can easily replace you, then your negotiation partner has a strong BATNA. And what is your BATNA? What happens to you if you can not come to an agreement? Make that comparison, to find out for whom a 'no deal' is the worst result.
If your negotiation partner's BATNA is not so strong, you could could consider walking away and break up negotiations. That might lead to a new concession on the other side. You have to make that risk-weighing. Sometimes you have to say 'No'. And realise that this could mean that it is really the end.
Unfortunately, we often say "no" without considering the result. Our ego is in the way. We see the negotiation as a game that we don't want to lose. So we say "no" and walk away. Only to wake up next morning and realise that we lost a deal that wasn't that bad and ended up with empty hands...