When Howard was a genuine star with legions of fans his ego could stand his comic image of a socially awkward, little dicked nebbish who was constantly menaced by Roosevelt toughs. It didn't hurt that his then wife was truly a beautiful woman with substance.
Now that you can fit his entire fan base in a movie theater and he's constantly mocked by his former fans his ego needs to be artificially inflated. For a similar example note how Gary has turned into a pompous douche since he fell asleep at work.
I think that this is how the divorce & Beth relationship came about:
*Howard starts cheating on Alison like crazy during the Filming of "Private Parts." Anyone who thinks that Howard never cheated is out of their mind at this point. He's given way too many indicators, including speaking in a very personal tone about how guys don't realize how much they're throwing away when they cheat.
The Sopranos dude confirmed that a girl he knew mentioned fucking Howard during this time period. Alison found condoms on his list during the Filming of "Private Parts." Howard mentions that Alison was crying over it. Lastly, remember Howard said he had a guy stand guard in front of his trailer so he could get some "sleep"? Yeah, right. No one needs a guard for that. A "do not disturb" sign would do. He was fucking in that trailer, and that's why he had someone on guard, so he wouldn't get caught by surprise.
*Howard first denies cheating when Alison finds out about the condoms. Eventually he admits it, maybe because of a guilty conscience, and maybe because he subconsciously wanted to continue living that type of lifestyle, and knew that by telling Alison, that would essentially end their relationship.
*Alison is livid, and immediately demands that Howard enters therapy on his own. Howard started seeing a therapist in 1997, and this would coincide with that timeline. Howard has mentioned he had his therapist's name & number on his desk for 5 years before finally using it. Why do you think he decided to start using it right then? Because Alison demanded it, IMO. Either that or Howard on his own decided to use it, in an effort to show Alison that he was really serious about saving the marriage (though he was probably partially torn, in that he wanted to continue having sex with other chicks).
*Alison becomes increasingly angrier, and can barely even be around Howard (she has mentioned before that if he ever cheated on her, she wouldn't even be able to look at him). Things are tense, so they decide that it's best for Howard to get an apartment in the city. They still commit to working on trying to save the marriage during this time, and so they enter couples counseling together.
Alison is under the impression that Howard will use the apartment to give each other space, but that they're still "married" and working on it, for all intents and purposes. He's not supposed to be fucking anyone else at this time.
*Howard starts having sex with others in the city during this time, while still "working on his marriage." This is why he talks about people having to go to his apartment with their heads down wearing hoodies.
*Howard sees Beth's audition tape for "Son of the Beach." I know that some people think that he was seeing Beth long before this, and he was the one that got her the audition, but I don't know that Howard would be that risky. I think the more likely scenario is that he saw her tape, got her contact info, and then began having sex with her.
This explains why Beth always adds on a year to their time together (she's not rounding, or else she would have had to have rounded by 14 months the first time she said this).
The really begin seeing each other around Mother's Day of 1999, not 2000. Alison is still under the impression that she and Howard are working on things.
*Howard starts falling for Beth, and this precipitates him really throwing in the towel & not fighting for the marriage anymore. Either that, or Howard admits to Alison that he's seeing her (& maybe others), and at this point Alison has had it. It's no small point that when they announced it, a separation agreement had already been signed. That means that Alison was fully prepared to divorce when that one year separation expired. Forget Howard's claims of "hoping we can work it out."
*Howard knows that the public is gonna be watching him at this point, so he tells Beth "let's cool it for awhile" so that she's not seen as the catalyst of the break up.
*He starts seeing other women, like Robin Givens, which upsets Beth. Howard mentioned that he told Robin that he wasn't gonna see her anymore because he found someone that he really liked, and he mentioned that he told Beth that he was gonna tell Robin this. That always struck me as kind of weird to me, as if Beth was sort of covertly pulling the strings. Just like when Howard used to always claim that Beth never once pressured him into marriage, but then he later admitted that she did. "They all do, Robin."
So Howard commits to Beth fully now, which is why Fred mentions Howard & Beth getting "BACK together."
*Howard & Beth set up that fake dinner party story, for their "introduction." This is why their stories change a hundred times about this. It was 100% fake from the beginning.
*The kids either know or sense that Howard was seeing Beth before, which is one of the reasons why they hate Beth. Perhaps just seeing the level of familiarity & comfort was enough to let them know that things had been going on for far longer than Howard & Beth let on.
Um, that was way long, so tl:dr version: Howard was cheating long before Beth, but Beth was the catalyst for why Howard & Alison no longer gave their marriage a chance of working out. He was seeing Beth long before the official time line, which is why their story changes so much.
Alison barely talks to Howard, that much is clear from what he says. I think that it must be hard for her seeing him with the woman he was cheating on her with (and he was cheating, since I think that Alison was under the impression that he was gonna remain faithful why trying to work on things).