I can relate, been there. All I can say is, just relax.
Think about it - if you've been in a relationship 13 years, you're used to someone being there and probably living with you, seeing them every day. In your new relationship, you've just had that really intense stage of being together every day. Having a few days on your own... Almost feels like withdrawal. Of course you miss her. It's nice that you do.
Me and my man used to talk for hours every day and see each other 4 days out of 7 for maybe 6 months. Then he had some life events where he needed a bit of space and me-time. That's normal - everyone has the right to space and this kind of intensity can't last forever. So he'd take a few days occasionally when we'd talk little/ not at all.
I missed him. A lot. I'd spend a lot of time thinking about us and decide in my head he was no longer interested. Then I'd text him and ask why he was no longer interested in me! This happened more than once. Luckily somehow he thought I wasn't a complete psycho and stuck around 😂
Don't be that person. Understand that everyone has different boundaries and needs, and right now she needs to focus. It's not to do with you. Well it is a little bit - she needs to think about work, not how much she fancies you, and also she doesn't want to give you substandard together-time when she is busy.
So think:
- This is not about you, it's about the way she focuses. Don't make it about you, not everything is about you
- She did the right thing by communicating it, it's not like she just disappeared off
- Keep busy. See friends you've neglected, do the things you need to do etc
- If you can't get her off your mind, maybe make some plans for next time you're together
- Remember - she has asked for a bit of quiet time to focus. You giving her that time shows how much you care for her, respect her wishes
- Think about all the ways she has demonstrated she's into you. You don't need reassurance of her love constantly, if she has spent all that time with you, it's obvious she's into you
- Remember also that as relationships progress and become more secure, you stop living in each others' pockets anyway, so occasional quiet time is normal in addition to couple-time
Looking back, in your shoes I wish I would have realised the following earlier:
"You don’t need weekly progress reports updating you on where she stands and how he feels.
If you put pressure on the relationship, you squeeze the life out of it and it stops being enjoyable and fun. Don’t push her for reassurance or test her to see how much she cares about you. Just realise she does and hold on to that conviction."
"A man cannot be a whole society to a woman and a woman cannot be a whole society to a man. 
Don’t expect her to be your best friend, therapist, financial advisor, source of gossip, business partner, and a hundred other things. That will eventually lead to a codependent relationship wherein none of you are growing. You will bore each other to death."
Spending some quality time alone often leads to better quality time together.