The best piece of advice I've ever received was stated plain and simple by a friend of mine in high school: “Don't ever make someone tell you they don't want you more than once." This can obviously be applied to various aspects of life where rejection is a possibility. Jobs, teams, friendships, and (of course) relationships. If anyone tells you, whether it be through their words or their actions, that they don't want you, the best thing you can do for yourself is believe them.
In the past year, I've seen so many girls who get clear, concise messages from their boyfriends that they're not invested in the relationship, and the girls just don't listen. It drives me absolutely crazy watching my friends – who are gorgeous, intelligent, awesome people – completely lose sight of their self-worth in the pursuit of a guy who could, quite frankly, not care less. Girls who won't even put up with me telling them their shoes don't match their outfit somehow find it acceptable to date a boy who doesn't value them, and stay with said boy after he cheats on her.
I'm just going to spell it out for you: there is no clearer way for a guy to show you how little he cares about you than by him cheating on you. If a boy (or anyone, this is obviously a gender-neutral life lesson) commits to you and then betrays your trust by hooking up with someone else, he's not right for you. Plain and simple. You deserve better.
I've discussed (and personally experienced) this situation from several angles, and there's really no remotely “good" way to spin it. It doesn't make a difference if it didn't mean anything, or he was “caught up in a moment," or if there were judgement-impairing factors involved (i.e. he was intoxicated and thinks that absolves him of any responsibility for his actions). Cheating on someone that you're in a relationship with requires the conscious acknowledgement of that person's place in your life and what they mean to you, and a conscious decision that something or someone else is more valuable to you – even if it's only “in a moment."
To accept that kind of behavior from someone who is supposed to appreciate, or even love you, is to set a tone in the relationship moving forward that says “I understand that you don't respect me in the way that you should, but I'm okay with that." If you find yourself in a situation where you're tempted to take back someone who has cheated on you, I encourage you to take the time to ask yourself: “Why?" Why am I okay with settling for a lower level of respect than I deserve? Why am I communicating that it is acceptable for my partner to disregard my feelings, and the terms of our relationship?
Obviously, every relationship is different, but I have seen an overwhelming trend of girls who blame themselves when their boyfriends cheat. Like, if you had just done something differently, or if you hadn't caused that one fight somehow his decision process would be different. But probably 999 times out of 1000, that's not the case. If someone cheats on you, it's because they made a decision based on their own insecurities that in no way involved or should reflect your merit as a girlfriend or a person. The best thing you could do in this situation is recognize this action for what it is: a sign that he (or she) is not emotionally ready to handle a committed relationship with you. If your significant other disrespects you and violates your trust, the healthiest thing you could do is respect yourself enough to walk away. Recognize that this person does not deserve your full commitment because they are not willing to reciprocate it, and leave.
I know it seems harsh, but everything honestly does happen for a reason. If someone sends you the message that they don't want you, you should hear it loud and clear. There are other fish in the sea who will value you and treat you right, and you deserve to find them. There is someone on this earth who will want no one else but you, and until you find that person, don't stop looking. If it was your daughter, or your sister, or your best friend whose boyfriend was cheating on them, what would you say? Would you tell them that it's okay, or that they shouldn't want better for themselves? Hopefully not. Hopefully, you would tell them to pick themselves up, brush the dirt off, and kick the scumbag to the curb because if he doesn't want to treat them right, another man will.