Look, you're not committing a crime, you're not trying to hurt anybody, you're not leading a double life. You're not affecting anybody else but yourself.

All you're doing is making the best of yourself and attempting to solve an issue so that it doesn't get on top of you and interfere with your personality or relationship.

If hiding it from her is making you anxious, thats not good for you and not fair on her.
Tell her and explain your reasoning.
Has she told you you look better recently?
If so, explain to her why.
Everybody needs a bit of support and explain that you would always want to support her in the decisions and choices she makes.

It might help you to have someone close to you that understands and that you can confide in.

If you carry on being anxious around her, you could easily drive her away.

You have two ways you can go about it.
You can tell her confidently and casually, like its no big deal and possibly then she won't see it as a big deal either.
Or you can explain from the heart and tell her what a big deal it actually is for you and get her sympathetic understanding.

Either way, I think you'll feel better and relieve some of the pressure you're putting yourself under.