Personally, if you REALLY REALLY like her and WANT to try to make it work, I would try approaching it along the lines of " I would like a new riding jacket (or helmet, or gloves, whatever) and am going to go shopping - would you like to go with me?" And, if she goes w/ you, while at the store, say "I really like you and I'd feel alot more at ease if you'd try on this jacket". If that goes well and you can coerce her into buying it - or letting you buy it for her, maybe you can incrementally get her to see the light.
On the other hand, if she refuses to go along with it, have the talk with her and tell her that you don't want to be hitching your life to someone who doesn't value their own as highly as you do, and break it off right there. She *may* have a conversion after a week or two and meet you in all the gear in your driveway asking to get together again, but she may well not. If not, consider that you've dodged a potential huge liability and go looking for another one - there are plenty out there...