Help my boyfriend of 2 years broke up with me because he is stressed about school?

My boyfriend and I had been together for almost two years.I am almost 25 and he is almost 26 we have a great time together, have the same goals in life, work well together and are going through the same masters program. We had been fighting off and on for the past month but it wasn't terrible by any means. Last week he tells me his feelings are distant towards me and he has been feeling this way on and off for the past 3 weeks and isn't sure why. Another factor 3 weeks ago he didn't p a major stepping stone in our masters program so this semester he has to do well in order to p. He has also been bringing up marriage saying hes not ready when both of us have agreed were not in a place in our life to consider that so he stressed himself over that too. He is now taking space to have time to himself but i don't want to give up on our relationship because we do get a long so well together. I don't know why he would give up on us just because hes stressed but he doesn't even know why he felt the way he did. He is also going away on a school trip this coming week and has a lot of work but once he comes back the following week will be our spring break. Should I give him space till he comes back and try and talk to him or should I wait for him to contact me. I want to work our relationship out because I think he got the wrong impression about marriage. I want to talk to him about the marriage topic because that was stressing me out and I also want to try and work things out I have written a letter about everything that was good and stressful about our relationship and hope it can make him reconsider. I just need some advice because I am really upset and feel I am losing my best friend. What are the best ways of going about space and what are the best ways to go about trying to talk things out with him?