A couple of times a month is not much at all. If you really like this girl and I am guessing you probably do, you need to try to work on it.
1. The next time you do it, make it all about her, give her the best orgasm she is likely to ever get, and this should hopefully make her want it more often to feel that experience over and over again.
2. Aphordisiacs, there are plenty of natural aphrodisiacs that could get her goung and make her want to do it more often.
3. You say she is tired alot of the time and that means no sex. Why not try to help her relax, run her ahot bath, possibly joib her in it and take it from there.
4. Tell her that your needs are not being met, and relationships are about compromise and see if you can meet half way.
Good luck.