Should I forgive him
I had been with my partner for 2 years we were living together, a month ago I was shown some Facebook messages he sent to a 14 year old girl, this girl had a bit of a crush it turned out with my partner, she was my daughters best friend. The police are involved and we are now living separately but he has says he knows he has done wrong and wants our relationship to work, even if he has to wait until my children are older (I have a 16 13 and 9 year old girls) he still wants to be with me, he knows he isn't aloud near my girls (police have bailed him) even though the social worker said she's 100% sure nothing has gone on and the police don't have any other evidence, in the messages it's obvious he has kissed her although he denies that and says he was stupid and wasn't thinking straight and I know his excuses are unbelievable, but he seems devastated to lose me, nothing changed in our relationship so I was so shocked when this came out. I have said I don't know as my older girls don't want to be near him, which I understand, and my children come first, but he says he will wait until they are older, we were due to marry next year. Can he be forgiven if he is sorry and knows what he has done is wrong, he is also seeing a doctor as he has admitted he needs help, is this salvageable?
I expect you are shocked, yes. In fact, I'd say that's putting it mildly.
Why on earth would any grown male - as such, undoubtedly perfectly capable of telling she had designs on him - even entertain any such advances rather than experience the usual, normal, giant alarm bells in his head and react accordingly appropriately (either ignoring or immediately putting straight or asking you to tip off the girl's parents)?
It's not unusual for there not to be any sufficiently firm evidence at that premature point, given that paedophiles are well known for incredible patience at biding their time when it comes to the "long con" of priming their victim (** and any adult guardian). But all that proves is insufficient lack of evidence for actual legal prosecution purposes, not lack of intent on his part.
** It's also highly common for paedophiles to take the same patient tack when it comes to one of the typical ways of preparing the ground: first forging what feels like a firm, romantic relationship with a target's mother and working hard on gaining her trust, specifically so THAT by the time any questionable incident occurs she'll find it too hard to reconcile this newer behaviour and image with the real persona she's falsely been led to grow used to and fall in-love with. (You said yourself that nothing had changed in (what you believed was) your relationship to have caused this development. So it's totally separate and independent from your relationship, isn't it. Doesn't that suggest it could have been there, waiting to emerge, all along?)
Madam, it's sounding convincingly like he's a paedophile to whatever greater or lesser degree. I get that this is hard on you in terms of your being called to turn your back on someone you THOUGHT loved you for you and for whom you'll have grown a significant attachment as well as based many a hope and dream on, one which is going to take some pain and hardship to get over. But  heartache and disappointment are par for the romantic course, no reason to tread where angels fear;  you've had this relationship only for 2 short years (not an 'irreversible' or long-lingering attachment, therefore),  the man/relationship is (ref your shock and total lack of any inkling) clearly false, anyway, in whatever way; and  where the protection of children is concerned - be they yours, their friends, your grown-up friends' and relatives' kids - CURRENT OR FUTURE - you simply don't take any chances, not if you're sensible, meaning, you're going to have to toughen up, bite the bullet, and remember that there are more fish in the sea than you could shake a stick at, including ones that DON'T represent a danger to children, yours or anyone's within your entire circles.
I'm afraid I don't believe he's devastated at the thought of losing you. It's too easy to try to cancel out the gravity of actions that are indisputably unacceptable in the lay sense if not the legal, yet which were wholly avoidable, AFTER the event with a load of blah-blah-blahs, including those the woman is obviously going to desperately want and need to hear. Anyway, convincing the woman he's heartbroken is another old chestnut in this situation, WHEREAS, that's the kind of bleedin' obvious consequence that anyone mature and healthy and genuinely in-love tends to feel devastated about *before* following through with, meaning, THEY DON'T DARE GO THERE IN THE FIRST PLACE. In other words, I love you and want you in my life and want never, ever to lose you, is as *DOES* (or this case, *DOESN'T*).
The thought of you and he dying as not just a potential but wholly likely consequence didn't stop him.
Personally, therefore, I'd say that not even the fact of his admitting he needs help and seeking it from his doctor (GOT INARGUABLE PROOF OF THAT, HAVE YOU?) could count as enough reassurance for you. If he has this clearly actionable tendency in him then it'll probably have been birthed/nurtured from a very early age, meaning, any psychotherapeutic intervention is going to be equally as long-haul. Plus, secondly, if a man isn't fit and ready to conduct a relationship at the same time as the woman, then even by that simple criterium HE CANNOT BE THE ONE because a relationship being destined to be and succeed is provable as such by all three of these criteria getting ticked at point of meeting: Right Person, Right Place, RIGHT TIME.
Furthermore, how do you think your children are going to feel about you ignoring all these major reasons against resuming this relationship; what do you think it's going to do to your present and future relationship with them?
I'd say the scales are coming down heaviest by far on the side of, cut your losses and walk away while you still can.
If you really don't want to err on the side of sensible caution then I'd say at least wait until you have sound proof of his seeking effective help, like he claims, and then again with proof that it's been insitu/attended for a reassuring enough length of time AND THAT THE CLINICIAN BELIEVES HE WARRANTS THE ALL-CLEAR.
Summary: Is WHAT salvageable?! A relationship with a man that either isn't founded on reality or contains serious issues/impediments thus is bound (when, not if) to fail anyway but in the meantime could destroy your relationship with not just your children but entire community? Answer: probably not, but Maybe hasn't even had time to become a consideration yet. Your only answer to him should be, 'Let's see, time will tell'.