I need a promise.

So, my boyfriend and I have been together for three years almost this coming June. However he recently joined the marines last January, making things a little more difficult than before but we always make things work. While he was in boot camp he would mention us getting married soon or him proposing soon. Which was last year. Now he’s telling me in a few years he wants to propose bc the divorce rate in the military is high. Which I understand not getting married rn, but I do want a promise from him or something showing me that he intends to do what he says he would do even before he enlisted. I feel like the next few years are going to be me waiting on him for almost everything which I can’t really complain about bc I do love him and support his career choices BUT my problem is, is how long am I going to have to wait? WELL. ACTUALLY. Now that I’m thinking about it I guess my real problem is, is him telling me things and getting my hopes up but changing the plan almost all the time. I don’t know how to get the promise I’m looking for bc I don’t want to be just boyfriend and girlfriend forever and be waiting on something that could not even happen.

Any advice?