Your hesitation in asking your mom for the ring is your already knowing what she will say and what your subconscience mind I trying to make you aware of if you are not already - you are too young and this is not going to end well.
You both are just babies and have a ton of life experience in front of you. And that natural progression to new chapters in your life is being over-shadowed by this perceived "love my soulmate" period. I am telling you - once this period wares off (trust me-it will) this is when those chapters you missed kick in. The meting new people, new things, the realization of just how little you guys have in common, the resentment in taking things away from me to cater to you and your needs, etc.
I was married at this young age and divorced as a result of what I explained above. I can't say for sure that you and your will experience all the crazy hot mess I or the majority of folks I know who married too young did, I can only say this - life is short. Be sure of every step you take so as to protect the good life you are capable of having. If it is real, you and she can stay engaged, promised, whatever for a while. Seriously - if you don't hit these speed bumps I speak of after say 5 years - go for it. I am only hoping to save you and her from the deep hurt that may be around the corner if you marry too young and it goes south. Think about it - make sure - be comfortable and ok with any decision you make about this - do not ignore your being afraid to ask. IT is telling you something
Good luck to you
Been there, done that ;p