At first, it will make no sense to you: how can I have this peace and joy even when I have denied myself, even when others persecute me, even when I don't get the results I want. You will experience that peace and joy in all circumstances, good or bad, if you seek Christ more and more. We know how to do that: the Church holds forth sacraments, prayers, devotions, approved renewal movements and entities, and great teachers of the faith to guide us; and the Church thus invites us to that joy and peace which indeed surpass our understanding. Our understanding sees reasons for sadness in self-denial, while the reality of continual conversion is that in the midst of suffering and even tragedy we still have peace and joy.
There are three "sufferings" that Sobrino mentions here:
a) when I have denied myself,
b) when others persecute me,
c) when I don't get the results I want
Christiany guarantees joy even when these three things are true. What it requires, however, is to distinuish between the levels of happiness. We aren't ever free of pain, suffering and disappointment. But, we must avoid insisting that God free us from suffering or failure or disappointment. If we become attached to anything in this world, no matter how seemingly noble or lofty, we will deprive ourselves of the joy we can experience. If we expect a certain result from a choices and don't get them, we are going to be disappointed and snuff out the joy that God plants in our hearts.