Military girlfriend advice please


So my boyfriend is trying to enlist in the army and he’s already qualified on the asvab. I’m trying to be supportive but the thing is I can’t handle him being gone. I literally am so clingy to this guy. I can’t go a couple days without seeing him so can you imagine 14 weeks!! I don’t know what to do bc I’ve tried telling him how I feel and he always just tells me to calm down and he’s not leaving yet. I have major abandonment issues bc of previous relationships. How do I get through this I literally am in love with this man but he is leaving and I know its for a short time but I can’t do long distance without talking to him or getting to see him. The thought makes me sick of him leaving. I cry every time I think about it. How do I tell him to stay or make myself happy with his decision even when he’ll be gone??