I believe that the reason, some partners think their significant others are cheating, is because they do not trust them.
Many people would agree that a relationship without trust, is like a basketball game without a basketball. It is an empty gesture.
The Ultimate Truth about partners, who falsely accuse their significant others with adultery or who keep on doubting the sincerity of their significant others, is that they can talk for days without ever being 'heard'.
Put yourself in your girlfriend's shoes:
No matter what she says, no matter how trustworthy she is, you simply will not fully trust her, you don't 'hear' her.
You could consider psychotherapy to work on your lack of trust in her (and probably your lack of trust in women in general) or it is time to split up because perhaps you are not ready for real intimacy.
Maybe you have had negative experiences with women, or with your mother. You may have been given the runaround in a previous relationship.
I really sympathize with you. If you are hurt, or cheated on, or abused, I deeply care for your suffering.
But your suffering is no reason to keep on doubting her and to hurt her, by questioning her intentions at every opportunity.
Now, Please join me in a thought experiment. Maybe this idea will resonate with you:
I want you to believe, that you have no self-confidence at all. You do not like yourself.
You do not respect yourself. You do not like the way you look. You are a shy guy, and you try to avoid attention and intimacy.
You simply can not, and will not believe, that a woman will ever be stupid enough to love you, to wanna be with you.
One day, a woman falls in love with you. She takes the initiative. She asks you out on a date.
After a while, she is very serious! She loves you, and she wants to start a relationship with you!
What will you think? How will you feel? How will you react?
You do not love, value, like yourself! This woman must surely be fooling around with you! She simply can not be sincere! How can you trust her? Maybe she wants your money! Maybe she is manipulating you! Maybe she wants her ex back, so she uses you to make him jealous!
Whatever her reasons, you will never really, and deeply believe, that she loves you!
Because you yourself do not think, you are worthy of love!
Is this something you MAY BE EXPERIENCING? Maybe?
If so, I hope, you will understand that, and seek professional help, to explore the background, the reason for your distrust.
I wish both of you to have a trusting relationship!