You can't email her, that's the coward's way out. You need to face up to her and tell her what you're feeling/thinking.
If you don't see a future in it, she needs to know that. It's only fair to her.
Are you able to articulate what the problem is from your perspective? If it's just that the "spark" has gone, then I might suggest that this is a perfectly normal thing in any relationship.
Having said all that, your story sounds remarkably similar to mine earlier this year. Although we weren't together quite as long as you, I knew I had to end it. Like you, I still loved her (still do) but I knew I couldn't give her the commitment she wanted (marriage, in this instance) so I had to pull the plug. Not that it's that easy, obviously. Took me about 3 months to summon up the courage. It's still hard work and the thought of being single at my age was not one I relished (I'm nearly 10 yrs older than you) but in an odd way, I loved her too much to string her along.
All I'd say is make sure you're 100% sure about your decision, then get it over with. Talk it over though, with someone who knows you both, who you trust to be discrete but who will give you a straight answer and tell you if you're being a nob.