The thing is, the letter is to simply tell her how things stand, from your point of view. It is not, necessarily, going to make everything right for you ~ certainly not immediately.
She has said to you:
'she loves me as a friend, .. having a relationship with her work load is too stressful, .. she hopes we can rekindle after university'.
She probably thinks that you need time apart, so that
you can accept that the romantic part of the relationship is over ~ at least for the time-being.
Maybe you will get back together again, after her course has ended. Maybe before then. Maybe never. Maybe you will just remain friends. But, even as friends, she doesn't have to keep in touch with you on a regular basis. She has university work to do and family pressures to deal with.
Remember, you said:
'she said we should be friends and see how it goes, and said she felt pressured'If you put her under further pressure, you may lose her completely. Don't
exert all this
effort. It won't help and it might make matters worse. She wants some time and space.
It may be intentional on her part, or she may be hurting. This should have been part of the reason for the letter ~ to say that you will give her space and leave her alone, but to confirm that you are there for her ~ at the end of the phone ~ if ever and whenever she needs you, because she was there for you, when you needed her. Then leave the ball in her court.
What, exactly, are you expecting from her?
You want her to phone you?
You want her to meet up as friends?
You want her to love you again?
You want her to not get involved with other boys?
There is no guarantee of any of this.
No-one can help who they fall in love with ~ and no-one can help falling out of love.
I think that love can be re-kindled, but if this is ever to happen in your case, you will need to respect her decision.
How old are you both?
Sometimes, around the time people go to university, they change ~ mature into adults, if you will. It is then that long-term relationships, which had endured through the teens, can draw to an end.
Not always, though. My husband & I met when we were in our teens & we have now been together, very happily, for nearly 33 years.
Good luck