Thank you for writing. As you are finding, having sex and making a commitment are two different things. Ideally, when sexual fidelity is important to people, the conversation about what they expect of each other comes before being intimate.
From your perspective your girlfriend “cheated”. I’m guessing that she didn’t think she had promised anything by having sex with you. I therefore don’t think what she did is the primary problem. Rather, it is your insecurity about your sexual prowess that is causing your pain. You are comparing yourself to other men negatively and you wanted your girlfriend to reassure you by being loyal to only you — even though you apparently never talked about it. Neither of you “lost” your virginity. You both made the choice to have sex and had different ideas about what it meant.
What happened is not unusual for a 16 year old. But you’ve now been torturing yourself with your doubts and anxieties for over 2 years. Enough. If you — or your former girlfriend — could make you feel secure, you would have done it already. Please get yourself some professional help. You deserve to have peace of mind. You need to get free of your negative thoughts about yourself as a lover so you can find the kind of tender and loving relationship you want.
I wish you well.