Is it normal i still love her?

I'll try to keep this concise.

5 years ago i discovered love at first sight with a woman from a different country and she was drawn to me. We eventually got together and had a bumpy start to a passionate longterm relationship. We worked out how to work as a couple and i did anything and everything for her because i was compelled to and we were great together, the excitenent, the laughing, the sex, we were best friends. I still remember the heart wrenching pain when we were apart or had fights or even if i was unrationally jealous about her.
Later in the relationship we had to find common interests (apart from the usual long walks, films, reading etc).
Now in the present we both can't help notice she has changed in the last couple of years. She's nearly always ill, doesn't bother with any hobbies apart from smoking weed, she's put on weight and has moments of depression, no sex drive as well as being homesick. She almost moved back to her home country but decided i was her life even though the relationship seems to be crumbling. I've tried to get her off the couch and take up new hobbies with me, stop smoking weed, get her sex drive back, exercise and eat healthily which always seems to only last a few days. She always tells me she thinks i need to be with someone more my type or fun and exciting but i honestly love her. Then she changes her mind and decides we can make it all work but with no real effort to change..
The confusing thing is i got caught up in a moment of passion with one of my close female friends who has liked me for a while and i was surprised how i recently felt strong feelings for her and how natural and good it felt until we got to the sex part and i had to stop because my heart ached for the one i love... Now my female friend is telling me to fight for my relationship. I am soo confused and hate to cause pain to anyone but what is the way forward here?