You have suffered enough at the hands of your H and his errant dick, so I'll try to be gentle, but please, pack this lark in already!
"However, her naveity and lack of understanding of real relationships is quite overwhelming "
Erm, YOU have a H that - under your nose - has shagged this girl, repeatedly, has brought her to YOUR home, had the audacity to stand there while you have welcomed her in, FOR HIM, and all the while he's banging her like a drum.
He's confessed, why? perhaps only because she threatened to bring you up to speed, not out of any sense of guilt or remorse - god only knows the bullshit he has told her about YOU to make her think that YOU and him are not tickety-boo and that she is justified in sleeping with him, knowing he has kids and a family.
YOU are sitting there coming up with reasons to justify his utter treachery, his kicking you when you are down, rather than manning up and supporting you, like GOOD Husbands do, you know, the ones that ARE worth keeping.
You are well entitled to take the moral high ground. You felt yourself slipping towards someone else, but YOU pulled that back. YOU know you could have done it and had an affair, but YOU stopped yourself. YOU pulled back.
Your H didn't. He chose to lie to your face, have sex with you while banging her on the side, and all of that going on and you never suspected a thing, you trusted him implicitly.
I see the word Naive, but it's not her that I think is being naive here love. sorry, but it's YOU.
did he tell you he always used a condom? how can you know if he did or didn't? he's lied through his teeth all this time, why not now.
Trust nothing but the truth that you can prove. Trust ACTIONS from this philandering cheat, not words. Get yourself tested, go speak to the CAB and find out what your entitlement to help/support/rights are, just in case.
The knowledge of what life will be if he doesn't pass muster will help you stay strong, and give you more power and bargaining position.