There's a lot of context to this story, so please stick with me 
DP and I met just Christmas 2017, we didn't become official until may 2018 as I wanted to be extra sure of it for DS sake. Just after we made it official, we spent 3 weeks apart - I took DS on a big family holiday and the day before we got back he went on holiday with his friend. All good, we both got home and carried on. Since then things have been great, until...
A few weeks ago when I recieved a Facebook message from a random girl of about 18 years old, 10 years younger than DP, (this is why I fucking hate social media) saying my DP had gone off with her older sister whilst they were on holiday last year. I was pretty skeptical as he'd always been so amazing, kind and lovely and just no suspicions at all that he'd been cheating. Plus, DP is not on Facebook so no clue how she found me. I did a bit of research before confronting him, found out that DP has a "secret?" Snapchat account that I didn't know about, which he was apparently constantly messaging other girls on when they were on holiday, so this girl told me. Why a guy who's nearly 30 would want to be on Snapchat I have no clue 
. Confronted him and he admitted he kissed a woman on holiday but it was for a few seconds on a night out and nothing else happened.
So then that brings us to now... I actually saw a Snapchat logo notification flash up on his phone the other night from a woman's name. At this point I'm feeling really paranoid/worried that something has gone on, so I waited til he fell asleep and went through his phone (I know, I know). I'd never ever do that if I didn't have some sort of serious suspicion.
So here we get to the actual point of the story!! At last. I go into the Snapchat account and look at the chat list. All girls. Screeds and screeds of girls names. I only had to look at a few to see that they were all from the week he was on holiday last summer. Really dirty messages, he was basically sexting other women the whole time, none older than 20. Bad enough, right? Then I look at one girl... Actually feel sick typing it, in a dirty conversation he asked her if she was "into girls" she said yes, he sent her a picture of me in my underwear - no bra, just pants. No face in the picture, he'd cropped that out. He said "that's my girlfriend", he knew what he was doing.
Confronted him immediately, told him it was absolutely unforgivable, I trusted him with pictures of myself, he's violated that. He begged and begged for forgiveness, told me it was at the very start of the relationship, to which I said but we were serious enough from about March but we waited for DS sake. He says he was scared that he knew we were going to be together for good, he knew we wouldn't break up and that scared him a bit. To be fair, since new year we've become a lot more serious, been getting along so so well and things had been amazing.
So as to not dripfeed, his family worry that I'm controlling as he stayed with me for 2 weeks when I had fainting episodes a few months ago (absolutely his decision to do so, I did initially say I'd go home to my parent's). They also are concerned about the fact I have DS and on my own, worry that I only want dp as a replacement dad - not the case at all as his dad is very much involved in his life.
I'm just so so hurt over all of this, I've always thought his family and I got along too, I bought all his siblings Xmas gifts, baked his mum cakes, I feel like a bit of a fool.
Anyway, sorry for the ramblings, thank you if you made it this far. Don't even know why I posted really, just don't know what to do about DP, needed to just get it off my chest to people I didn't know.