Second thoughts about my engagemem

I just got proposed this Xmas but things have been getting worse and I don't think saying 'yes' was the correct answer. I've been with boyfriend (current living with) for 4 years and since we moved in together we argue every week. We argue over the most stupidest things to saying the nastiest things to each other. I'm trying so hard to keep the peace between us even though sometimes its not even my fault..I understand all relationships have problems but this is becoming too much.

I love him soo much and in a crazy way I know he loves me too. if he knew I was having seconds thoughts it would kill him but idk what to do anymore....