@XDownwiththissortofthingX
As I've already mentioned, "fundamental honesty" is noble in principle, but it isn't a basis for workable relationships between human beings. Internet forums, Mumsnet included, tend to be hugely fond of straight forward black or white thinking, with little nuance, and no apparent comprehension of the fact that humans are flawed, they are not black or white, humanity and life exists in shades of grey, and there are times where it serves no worthwhile purpose to be fundamentally, brutally honest.
Try it, but if you adopt a policy of 100% honesty, at least have the decency to operate that policy literally, and then see how far you get. Tell your partner that no, you don't want to watch TV with them because their taste is crap and it bores you to tears. Tell your partner that no, the meal they slaved over for hours wasn't great, in fact, you've had far better on multiple occasions. Try it, see how far it gets you, and how quickly you realise that not only is 100% honesty not a good thing, lies and dishonesty are actually essential for human relationships to persist for any length of time whatsoever.
This is such nonsense, and a perfect example of the kind of black and white thinking you're supposedly against.
Leaving aside that as a previous poster said, a small white lie about a bad meal is quite a different situation to hiding the fact that you've cheated, it is perfectly possible to be honest without being brutal in almost all circumstances, and if you can't do that, honesty isn't the problem, the problem is your limited communication skills.
"Hon, I'm not a huge fan of Survivor, but I'm happy to read on the couch while you watch!"
"Hmm, Love Island isn't really my thing but what do you reckon about Ted Lasso?"
"Babe, we have so many things in common but you know our taste in TV isn't one of them. How about we pick a movie or play cards instead?"
"Thank you so much for the effort you put into that dinner! The potatoes were great. My turn to do something special next week."
"To be honest, sweetheart, I struggle to eat curries that hot but would love to try it again sometime with less chilli."
See how it works?
The OP's honesty to her boyfriend could sound something like this.
"Bob, I have something really difficult to tell you. When I was away on holiday, I got very drunk one night and slept with someone else. He is not someone I'll ever be in contact with again. I made a really bad choice under the influence of too much alcohol, and I feel absolutely terrible about it. I'm so, so, sorry to have to tell you, but I felt you deserved my honesty and to be able to make your own choices about whether or how we go forward together.
"I want you to know my cheating was not at all about you or whether I want to be with you. I care about you a great deal, and this was all about my own insecurities and issues, and not any lack on your part. I know you'll be feeling really hurt, but I really hope we can work through this. If I'm honest, I've been finding our long-distance relationship really hard, and while it's not an excuse, I think it played into my bad choices. If we were to stay together, I'd like for us to find ways to be together more. I also see I've made similar bad choices around drinking before and have tried to work through it in therapy, but clearly I need to do more work. I've realised I need to stop drinking because I just don't make these kinds of choices sober. That's a commitment I'm going to make whether you want to continue this relationship or not."