Not sure what to do

My fiance and I will be together for 4 years in June, we've been engaged for about 3 and also have a 10 month old together. The other night he randomly mentions that we will be getting married in a couple months. We always had this idea to get married on our anniversary date at the courthouse and later down the road renew our vows and have the big sha-bang wedding when we can afford it. Well the only problem is.... I'm not ready to be married yet. I only turned 21 in January and he turned 22 this month. In fact, I don't think I ever want to get married. I'm just so lost on what to do. I don't want our relationship to fail because I don't want to yet, or whatever else can happen when I finally admit to him I'm not ready. Its just so hard because I don't think he would understand why i feel that way. I don't even know if I want to go to college or not, or continue being a sahm. Getting married is a major commitment but where I'm at in life right now it just doesn't seem right...