You may be trying to make too big of a jump; if money is an area that you really struggle with, declaring that you're wealthy will just remind you of how wealthy you aren't. Maybe what you could do is, when you hear your inner dialogue 'correcting' your affirmation, you could take a moment to acknowledge that that is just your old way of thinking resurfacing. You've only been recently introducing these positive thoughts of abundance into your way of thinking, so it makes sense that your old habits of thought that you've been thinking for way longer are going to take some time to dissipate. It's only to be expected really, just tell yourself 'that's just the old me piping up, not the new me.'
Personally speaking, I think affirmations only really work if they actually cause you to feel what you are speaking. If telling yourself that you are rich doesn't actually make you feel abundant, look for examples of abundance in your life and appreciate them. If you focus on abundance, you will attract more. Also, something I read in a book that I found really helpful was to sit and think about how much money exists in the world. This is the book it's from, it's really short it's more of a pamphlet honestly, I really recommend it for people who have scarcity complexes in regards to money: