Don't be hard on yourself. Cousins have gone through this type of thing for as long as there have been families. However, you are very wise to see that you are the one who needs to set clearer boundaries because you are older. It sounds like you need to sit down and have a real conversation about this. Although it won't be easy it is more likely draw you close together in the long run, more so than anything physical which could be very hard on both of your lives.