It isn't supposed to be fun.
I have had kind of a lot of "making it fun" references to practice lately.
I find that for my friends and yoga peers, I am happy to encourage them to do what is necessary to "make practice fun." I generally like the old adage, "if you're not having fun, then you're doing it wrong." But I realized, during my practice today (which was not particularly fun), that I encourage fun for my friends. I encourage fun for those struggling practitioners who are not "my" students.
When I am faced with a fellow practitioner who is genuinely looking to me as a mentor, I never ask them if they are having fun. I never encourage them to modify the practice or change their approach for the sake of fun. Maybe I encourage change for realistic daily maintenance, or to be considerate of injury, or to work toward a specific goal. but never for fun.
Because I don't think this practice is supposed to be fun.
That doesn't mean it can't be fun (sometimes). That doesn't mean I think you shouldn't enjoy it (even when it's not fun). But that's not why we do it. We do ashtanga yoga because we seek discipline. We seek challenge. We seek to be reminded of impermanence and our body's fragile and transient nature.
Not because we want to have fun.
We do this practice because we hope to face (and subsequently lessen) the suffering that inherently exists as a part of our perceived reality. We practice the series because we trust that it will allow us the opportunities we need to do this. We trust our teacher because we know that he or she will be able to determine what level of suffering we can take.
If you need your practice to be fun, then maybe you aren't facing something that the practice is trying to teach you. Enjoy it when it is fun. Suffer and face it when it is the time for it. Some suffering is inevitable and it is there that we learn the most. So, I challenge you, don't change things to make it fun. Just do your practice, all is coming.