I do exactly the same and it's caused me plenty of heart ache. In hindsight it's so clear when I've overstepped the line.
Ultimately I think one needs to love oneself more. I know I need to be more comfortable with my own company. To not consider any person as the center of my universe. I know it's easier said than that. Try plan your day around what YOU want to do, including being around other people. Make a conscious effort to limit your time with this person and spread your emotional needs out