Depends somewhat on the particular negative. We can claim with certainty that dinosaurs are not alive today, even though there are theoretically places that are not sufficiently explored where, in principle, a dinosaur might live. We are well aware that the fossil record is globally consistent and demonstrates the timing of their extinction, as well as the lack of any fossils of dinosaurs since then.
Similarly, I can claim with confidence that a force does not exist in the universe that would allow me to teleport from place to place instantaneously, because such a force would violate many of the known laws of physics.
There are various types of god-beings that cannot exist because of a logical contradiction. An omnipresent entity, for instance, would be always detectable if it had any measurable effect on the world, but no such measurable effect can be observed. If it is not capable of a measurable effect, then it is indistinguishable from not existing.
Other types of negatives are not easily proven. That is why scientists will say things like, “Based on 3478 measurements, there is currently no evidence to support the existence of higher values than those reported here.”
I am nearly twice your age. Pretty much everybody has been through a version of what you are experiencing. Two things you must know.
1: You had a relationship that was valuable enough to you that losing it hurt. That is special in itself, and you need to consider the good thing that you had. Seeing a concert is ephemeral, but you remember the music and performance positively. Think of the relationship this way, also.
2: Give yourself time to mourn. Mourning is natural, and it is a process, but there is an endpoint. Many have walked this path and come out on the other end.
You are young, and you will have many more experiences in your life. Some positive, some negative. But your life is far from over.
Take a day, go for a long walk at the beach or park or nature trail. Cry, weep, wail, knash your teeth. Then move on.