My husband and I aren't being intimate.?

My husband and I aren't being intimate. My husband wants it all the time, but me on the other hand could live without it. Its just that I feel very down and hopeless like if I missed out on my youth. We did get married very young at 18. And have two beautiful boys a 2yr old and a 4 mnth old. I love my kids to death and don't regret having them they really are the best thing that has ever happend to me besides my husband of course. I am a stay at home mom right now. Its just sometimes I feel down,sad,hopeless I don't even want my husband to touch me and very alone even though my husband is very attentive and loving towards me all the time. Its just I really don't want to be intimate and I'm scared if I don't give it to him he will get it somewhere else. I just don't no what's wrong with me.