Most Helpful Girl
Just be honest, and ask to talk with him in person somewhere private (like your room, or the living room when no ones home) and let him know what you've been dealing with.
Tell him you want to share this with him bec he's an important person in your life, and be honest. Explain to him how it feels being apart, and remind him you do need your space, but you are also struggling with this anxiety that makes things hard.
He's your boyfriend, you can trust and share whatever you want with him! Especially if it involves him. So have a relaxing evening in, unwind and get a good chat going, cuddle up and ask him to talk about something important. Then explain
I'm sure he will be VERY glad you told him this, and it'l help him understand what you're dealing with too, and hopefully he can find ways to help.
Also, maybe keeping a picture of him in a locket necklace will help you? Or have him send you a good morning message each day through voicemail, that you can replay through the day when you need to hear his voice. Take one of his t-shirts to wear when you miss him, and ask for his bracelet to wear as a comfort piece from him.
You can even ask him to spray some of his cologne onto your pillow and blankets when he's over to help you sleep! My boyfriend does this. I also stash his hoodies in my bed and wear them at night. He knows, but he doesn't mind :)
I hope you both can work together and get through this!