I'm going to agree with most of what is being said in this thread. She is becoming distant. You said this. What other reason would there be. Whether or not she is currently cheating is irrelevant. The relationship is essentially over
You are going to rationalize this in any way you can because that is what the human mind does. It tries to convince itself of what it wants to believe.
You need to realize that the face she has been putting on (yes for one and a half YEARS) is NOT who she really is. There is a completely different person deep down in every girl. Once you being to see this in every girl, it can be depressing and shocking and eye-opening. Your little princess is NOT an exception. I'm not saying that she is a cheating wh0re NECESSARILY. Just know that she is perfectly capable of the following things:
1) Putting on a pretty little innocent face while lying to you
2) Pretending everything is alright and telling you shes okay when it's not alright, and shes not okay.
3) Talking to and/or seeing another guy behind your back. You are blinded by her love and are unable to think rationally and clearly.
Girls are incredibly, horrendously sneaky. There's a saying that goes "Girls cheat as much as (or more than) guys...they are simply better at hiding it. Never forget the fact that this precious, lovely woman who sucks your d!ck and calls you sweetheart and cuddles with you is capable of coldly and suddenly dropping you at a moment's notice. THIS GOES FOR EVERY WOMAN. Your girl is not the exception. She can do this because she takes the blame off of herself for what she is doing. In order for her to still see herself as a "good" person, she rationalizes ending the relationship by looking at every little tiny thing that you've done wrong, amplifying it in her head, and then saying "yea, see he's not such a great guy. I was getting bored....i deserve better". She turns the image of you into a distorted, worse version of the great guy that you are in order to rationalize leaving you. "Then why would she leave a great guy?"
This could be for a number of reasons, but primarily because she has lost attraction for you. Now a part (or most) of this is very likely your fault, and you need to come to terms with this. You probably grew complacent, doing the same activities, having the same routine sex, etc. etc. It could even be (as you said), you becoming too AFC and hanging out with her too much, or her sensing that she has the power in the relationship.
You are trying to rationalize. Stop it. You KNOW you're trying to rationalize. You're smart enough to have gotten here and posted on this board and realize you messed up. So i know you're smart enough to REALIZE that you are rationalizing this girl. "She isn't acting shady either" "She isnt hiding her phone"..do NOT underestimate girls. They will text guys RIGHT in front of you. The safest place is often right next to the danger. She is a sexual, cunning creature. She is capable of all sorts of things. Now im not NECESSARILY saying she's cheating.
But here's the tricky part...What is cheating? To me, and probably you, there is a clear line. Kissing/cuddling other guys/girls is NOT okay. Meeting up alone in an isolated place with guys/girls who arent clearly friends is NOT okay. Any more than polite banter via texting (AKA flirting) with a strange guy/girl who isnt your established friend is NOT okay. But to girls it is different. Cheating is as far as you can rationalize. Some girls would say that grinding on a random guy is not cheating. Some would say that kissing a guy is not cheating, especially if you didnt find out about it happening. Hell, i've even had a girl i know (i knew her boyfriend, he was a cool dude) drunkenly say "it's not cheating if i dont have sex, RIGHT?!" and then laugh about it. I've heard a girl say "well it's okay because i didnt love him any more." WHAT. What kind of FVCKED up logic is THAT?
Even the "good" girls who dont "actually" cheat (aka dont makeout or sleep with another guy), still usually have a guy on the back burner set and ready to go for when she dumps you. You think she just met this guy and decides to jump ship? NO. she's probably known him at least a few weeks, hung out with him a couple times, been texting him and deleting her conversations (or telling him not to send anything incriminating that you might see, so you label him as "YOU SAY HE"S JUST A FRIEND OH BABY YOUOUU"), etc.
You need to play devil's advocate for a bit and say "okay, well what IF she was capable of this" and then try to analyze her behavior from a clear-headed point of view (which is probably impossible with your feelings right now.)
Regardless of all this, the fact remains. If you chase her you will lose her. Hammer this into your god-damn, fvcking head man. You're being told this by nearly every distinguished member of the board here for a reason. It is the truth. Do NOT text her, do NOT talk to her unless you need to, and for fvck's sake do NOT text her and ask if she's okay!!! I have been there. I said the same exact fvcking thing. I asked her if she was okay. I asked her "are we cool?". Guess what happened? Go on. Guess. She said "yea, we're cool", kept withdrawing from me and then dropped my pathetic, sorry a$$ the next week.
Honestly though, there is no-one that can make a man change his own mind besides him. There is no one that can change what a man wants to do aside from himself. Regardless of how many people tell you to drop her, you probably wont. It sucks, but it's not a bad thing because it will help you to realize in a way that you probably couldn't have if you didnt mess up.