How to prepare for a hurtful confession

Tell her as soon as possible.

You've stated that you've had to tell more lies to cover for the original lie. That suggests the longer you wait to clear this up, the more lies you'll have to tell in the meantime, which will make things worse when you do confess. There's a huge difference between exaggerating the truth when you first start seeing someone and repeatedly lying to them as you become closer. Bad news does not get better with time.

Now for how to tell her. Pick a time when you're both calm and have some available time. Tell her you need to confess something, and then tell her directly. She'll probably ask why you lied and why you're choosing now to come clean. Be honest. Hopefully she won't be upset about your actual income and will only be upset that you lied, and the best way to start to heal that hurt is to be completely honest.

She may or may not forgive it. There's nothing you can do to ensure she will move past it. This will give you the best chance. Trust me, I have experience telling a dumb lie at the beginning of a relationship that I thought would never become serious, and I waited a long time to come clean. It did not go well, and most of the hurt was caused by all the lies I had to tell to cover up the first one. Don't make the same mistake of turning a problem into a much bigger one.

What do you want from your relationship?

Do you really want your relationship to be based on money, power or something like that?

In some relations, maybe yes. We're all doing "fake it till you make it" but it's working with something you'll achieve sooner.

Example:

I'm a Python developer. I'm good at it. I'm applying a job which requires PHP skills. I say "Yes! I'm good at PHP!" but I'm not. What I'm good is Python. But both Python and PHP are scripting languages and I've the fundamental knowledge. Therefore, in a few months, I'll be at least over moderate with PHP too. I've just didn't studied until today. Now I need it, I'll learn.

Another example:

Have you heard this song? "Ah yes!" (But I didn't, I'm going to listen it now.)

Do we really need this kind of things in our life? I mean come on, just tell you didn't listen that song but you will. You won't lose anything by being honest.

About the work example, I trust myself. I've fundamentals. I'll learn. What if I won't learn PHP? I'll get fired. No problem for the company.

These are just basic examples to show that lying eventually will cause problems.

What can you do right now?

Ask yourself. Why did you lied?

Possible reasons: He/she's so great, you feel like you don't deserve him/her. He/she's too good for you, smarter than you, has better looking than you etc.

Confess to yourself.

And then go tell him/her. "I've lied about this because I feel like (....) about my income, I feel like (....) about you and I feel like (.....) about myself.

Develop self-confidence. Build your relationships upon respect, love, honesty. Not little games, puzzles like they teach on TV.