I keep my private life to myself in work: now I'm nervous about telling colleagues I'm engaged

Don't worry, you're not odd at all.  Plenty of people prefer to keep their work and personal life separate, it's perfectly normal.  Your colleagues do know you have a boyfriend from what you've told me, so they won't be totally surprised anyway.

Go in to work wearing your ring (if you have one), a big happy smile and tell them about the proposal - they'll be delighted for you.  People love to hear happy news and after you've told them you'll wonder why you were so nervous about it.

If you'd like to be more open and share more with work friends then telling them about your engagement is a great way to start.  Ask people for coffee, tell them about your holiday, share little bits of your life and others will respond.

Congratulations on the engagement and really enjoy this happy time.