Afraid to break up

Have been with my bf for 5 years and I'vent really known anyone before him. He's my first relationship. We have a very healthy one and he's kinda perfect (sweet, would do anything for me, same humour as me, smart ) but i dont know why it's not enough...i feel like I need more, experience more. The best option would be having an open relationship but I dont even ask as he would hate it and I'll lose him the worst way. We're living together, I am afraid to lose him, to lose my best friend and also a comfortable relationship ...but I feel unhappy and frustrated as the same time. I dont know what to do :(