i need advice from all different people please!

caitlynmae • 17 years old
so my current boyfriend, his past relationship was one of those where you meet at high school in year 7 and have a thing on and off from then onwards.. i had one and would say i thought I was in love with him anyway.. 
my boyf has broken his phone atm so uses my phone for all his like social media and that, on his Instagram he still has the "6 months with this special lady" photos on his Instagram with him and his ex i was totally fine with it the first time I saw it but that did prevent me from answering his messages when he first tried to talk to me as I thought he was still in a relationship .. then he signed into his Instagram on my phone a few days ago and checked his direct messages he had kept the messages from his ex girlfriend the last message reading "i love you" from like 100+ days ago and that was that all sorts of shit went through my head like I swear you delete that crap when it ends so you can explore into a new relationship without your last one still lingering over your shoulder? fyi he cheated on his ex girlfriend at the end of their relationship so i have. I clue why he would even want the shame of remembering it all on public to everyone? 
it makes me feel like a mug still seeing that there I'm not one to care or get jealous but seeing as my last relationship I would've got a hard beating if I'd kept anything like that on my phone..plus my ex asked me to sign into his account once and there right in front of me was messages from him and his ex talking about how "last night was good" ectect. my trust issues are crazy, i love this boy with everything but I'm scared I'm just a silly girl that's their to replace a girl who won't get back with him because he cheated iden 
I confronted him about it just after I'd seen the messages and he told me I was being stupid and silly and its from 100+ days ago.. I said that doesn't matter why do you have it you are in a new relationship now.. He said ok give me your phone I'll delete it I said no don't leave it it's fine purely because I didn't want to controlling and look like a physio bitch 
please someone give me feedback?