long term relationship problems

Im young and I've been with my so for over 2 years now. We are engaged and things were alright but for the last few months its just seemed blah. We are both really young but it feels like we have been married for like 30 years. Sparks gone, butterflies are loooong gone and we both dont know how to spice it up again. I feel weird because i feel like this is the time where im supposed to be excited to be in a relationship and im scared he may lose interest in me. Any advice?