How do you spice things up in the relationship?

My bf and I have been together for 2 1/2 years and things have been great. He’s an amazing guy and I adore him and I’m very much attracted to him, but we’ve fallen into a routine. We’re broke university students and can’t afford to go out every weekend. Our sex life isn’t how it used to be. We used to do it 3x a day sometimes and now we go weeks without doing it because I’m never in the mood. I want to be but I’m always too stressed about life problems to enjoy sex. How can I solve this issue? Is anyone else experiencing this too?