You're doing great, BM. Please take everyone's advice on here and call 101. Even if they never have to act, it will set your mind at rest knowing you have logged everything and got the benefit of their advice.
YY also to PP who advised he may escalate, please do not be scared into placating him further. If he "turns up" anywhere you regularly spend time, walk away without a word. If he gets in touch threatening suicide or self harm, call 999. If he comes to your home, don't hesitaate to call the police.
I know it all sounds extreme but these things really do happen. I spilt up with my boyfriend when I was 19 for similar reasons to yours. He went from being a very nice (albeit clingy) person to a threatening, stalking nightmare. I took him back numerous times because I believed his threats of suicide and felt so guilty for hurting him (no MN to advise me then). The final straw came when he came to my house on the well-worn pretext of retrieving possessions and, while I was getting an item he'd asked for, went into my bedroom and destroyed some of my things in a really sinister way (beheading my stuffed animals saved from childhood etc).
It took intervention from my family to finally scare him off. I still felt guilty and worried that I'd hurt him irreparably, but he had a new girlfriend soon after and is absolutely fine to this day. Your ex will be too.
Sorry for the essay, I just want to spare you the stress and guilt I had. You're free now, please don't back down.