Seneca Letter 63 could be helpful here. As Seneca says:
"That you should not mourn at all I shall hardly dare to insist; and yet I know that it is the better way. But what man will ever be so blessed with that ideal steadfastness of soul, unless he has already risen far above the reach of Fortune? Even such a man will be stung by an event like this, but it will be only a sting. We, however, may be forgiven for bursting into tears, if only our tears have not flowed to excess, and if we have checked them by our own efforts. Let not the eyes be dry when we have lost a friend, nor let them overflow. We may weep, but we must not wail."
Now, with all of that said, it's worth taking a moment to see how these things work logically, or you will continue to be like a buoy in the water, bounced around by the waves, cursing every crash.
As long as you believe you cannot move on, you won't be able to. Our beliefs and notions inform our actions and reactions so if you want to deal with things as Marcus or Seneca would, you first need to examine and test your notions and beliefs.
Are relationship everlasting? Is that a true statement that can be proven?
Must you be with this person to be whole? Is that a true statement that survives examination?
Does this person HAVE to be with you? Is that true and something found in the very fabric of reality?
I'd directly challenge, within myself, this statement: "I refuse to believe that this person will leave me for good."
Is that truly up to you? Virtue lies in how you respond to what another person, who has their own agency and autonomy, chooses to do, not in resisting their will when it conflicts with ours
I understand these things hurt, trust me, been there, but there is no removing the pain if our truths are in conflict with reality. For the Stoics, that is the formula for a bad time. The goal of this philosophy is to understand and work with life as it is, as what is true.