The way I feel about my boyfriend.. HELP!

I've been with my SO for 3 years! I'm 18 next month and he turned 20 in June.. I've been having baby fever and he knows that and I can't stop talking about it and I've been thinking about engagement a lot.. He's not ready for it and I respect that but I still can't stop talking about it and get upset whenever he ignores the topic and tells me how he doesn't want that till a few years and YES it is the right thing and he thinks proper but ever since this it just makes me feel rejected and I'm starting to feel different about our relationship I guess? I know I love him more than anything and I wouldn't be able to live without him but I think it's all because I'm so into wanting a family with him and wanting to be his fiancé that I'm feeling this way.. Does it make sense?! I need advice because I'm starting to scare myself a lot.. I'm happy I really am we don't fight a lot and he does everything he can for me but one way I'm unhappy is again because I want a family and to be engaged which is so foolish of me.. I'm scared of the way I'm feeling because never in a million years would I leave him but I know if he proposed or I got pregnant I would start to feel the same again.. Did anyone go through this?!