Less sex apart of being together for awhile?

My partner and I have been together almost 9 years. We love one another and have kids. He works a lot and I stay home. Since our last child (2 years ago) our sex life has been less active.  ... But when we do have it... Its amazing. 
Is this a normal thing to not be as sexual when you have been with someone this long? I realized we are having sex once a week sometimes its 2 weeks ! I used to be this sexual person and now not so much... I just don't want it like I used to... Tho I do want to be the old me again! Help!