I don't know if I should be done and leave(sensitive content)

So I 28 f and my situationship 33 m have know each other for 12 years and have been living together for 3 years. We have talked about marriage for a year. Just recently I had found out that I was pregnant, but unfortunately I had a miscarriage. We talk about trying for another but now he has become more distant. I basically sleep next to someone and always feel alone. Just recently found out he gave me an std yet still denieds he is cheating because apparently we only live together and we are not together. Every time I try and leave I get the man I always want but it only last for so long and it's taking a huge affect on my mental health. I literally cry myself to sleep or get so exhausted I have to force myself to sleep. And sorry if this is long and there is definitely more to the story but I need help and also needed a place to vent about my situation. Thank you for reading this far if you have any any advice is greatly appreciated but plz no slandering 💜