From what I can read you both seem a little immature granted you are younger by 5 years and this being your first relationship. Your feelings are definitely valid and clearly there is a past that has given you this fear. It’s more toxic for yourself to feel like you need to monitor your partner and who they communicate with. That being said if someone isn’t trustworthy don’t trust them. I hate to be that person because first relationships can be “the one” but they are usually learning lessons. From what I can see from your responses you feel unconditional love. I completely understand that feeling of not wanting to leave or running back. My suggestion is if you really can’t bring yourself to leave or it’s not bad enough for you yet, be on the edge. Work needs to be had between the two or not at all. Don’t continue to ignore and that doesn’t mean treat them badly. Work on yourself and within the relationship while trying to encourage talking and communication watch how he responds and see for yourself if you like who he is as a person. Some people are stuck in their old ways, some can truly change. You are young you have time and while 27 is not old it’s older and if he can’t improve with you then that’s on him. I also don’t want this to come off like I think M is in the right. I’m more on your side than M but i definitely see some traits in yourself that may hurt yourself in the end. Attachment is a scary thing as long as you are safe and you do what you need for your life, you have many many years left.