Is there any justifiable reason to cheat on somebody
No.
as opposed to just breaking up with them or sorting out whatever issue you have with them?
In a normal relationship, cheating is never justifiable. There may be some unusual hypothetical someone could craft, like someone in an arranged marriage who's too scared to say anything or some other situation I haven't thought of, but in a normal relationship between two people, there's no justification for it. It's unethical, full stop. Anyone who does it is a crappy person.
Have the decency to tell someone if you're not interested in them anymore. And if you are interested in them, then don't cheat on them. If you want an open relationship, discuss it with them, and if they're not comfortable with it, either respect that or find someone else to be with. This stuff's not rocket science, people are just stupid or awful. Or both.
I get that sometimes people do stupid things in the heat of the moment, but that doesn't make doing it any less bad. Acknowledge that you did something really awful to someone who presumably loves you, do your best to do right by the partner you wronged, and make a sincere commitment to being a less awful person in the future.
As someone who was just cheated on after being in a "devoted" relationship with this person for years, I can't for the life of me find out why somebody who you care for so much and do everything for could do something so hurtful.
I'm really sorry to hear that. I have a friend who cheated on his wife and I asked him what the hell he was thinking. His answer was that he
wasn't thinking; the situation presented itself to him and he went with it even though he definitely knew better.
I can't speak generally here, but I understand why that happened. Going with the flow is natural, thinking with your sexual organs instead of your head is natural. Doing the right thing in this kind of situation requires a conscious effort, one that I would have hoped my friend would have made but I can at least process why he didn't. A lot of bad things happen because nobody decides to stop them; this is true outside of relationships as well as in them (look up the bystander effect for an example of how this sometimes shows up elsewhere). I've had relationships (plural) of my own fall apart because I didn't put in the effort to make them work.
I don't know the circumstances behind what happened to you, so I can't speak to your specific scenario, but I can tell you at least in some cases it's not a result of malicious intent. There are certainly some cases where it is; that happened to my brother. But I think at least sometimes people do bad things because they're too weak to do the right thing. Best you can hope for is that they realize how crappy they've been and decide they want to work on becoming a less crappy person.
Have you been cheated on before?
Sort of.
Did you ever get over it, if you did.. how long did it take before the pain subsided?
I was mostly over it in a month or so, but it still leaves a bad taste in my mouth years later. I've had worse things happen to me since then, though, and I got over those too. Life goes on.
I will say that while I don't really think about it much anymore, the lessons I learned from that situation still drive my behavior today. I'm a lot less trusting of other people than I used to be, at least when it comes to relationships. On one hand, that might be a bad thing because relationships do rely on some degree of trust, but on the other, experience is what it is. You wouldn't
have to trust people if the potential wasn't there for them to betray that trust.
Did anything alleviate the pain and/or SEVERE depression you felt?
Time, focusing on other parts of my life, and eventually reflecting on the situation when the pain wasn't still fresh and raw. Thinking about it early on isn't helpful because you're too emotional about it. That will pass, and then you can think about it more objectively.
Edit: oh wow, didn't realize this was a two year old topic that someone bumped. Hopefully still useful to someone.