My boyfriend wants to serve for 4 years in the military

I’m absolutely terrified. I have no idea what to expect because I never thought this is something I’d have to go through. Can someone tell me what it’s going to be like? How can I stay strong through it? He’s my support system. I’m used to seeing him everyday. But this is what he wants to do, and I love him so I’ll support him. I’d really appreciate some advice