Next, live like a pauper, saving 60-70+% of that income. Spend your free time doing exactly 3 things 1. working on a side project 2. spending time with your girlfriend. 3. exercise (never count this out, no matter what your life circumstances otherwise)
Then just wait 5 more years. You will either have something going from your side project or you will have enough money saved to legitimately work full time on something without worrying about bills
As far as the idea, go for something as boring as possible. Preferably something that someone else is already paying money for but that the incumbent product is shitty or lacking in some major way. Fix it or make it signficantly cheaper. Business is just a wrapper around value.
Create value, get paid. That's how capitalism works.