First, I’d like to welcome you to the forum.
Here are some of my thoughts on this matter. To “rigorously follow” Catholic norms and practices lacks meaning if I do not have a relationship with Jesus. My first question would be why are you trying to do these things if not to grow closer to God?
Yes, there are scientific benefits to fasting and giving up unhealthy things, but we do these things to grow closer to the Lord, Who loves me, even if I did not do these things. When we give up worldly comforts and habits, we allow God’s grace into our lives. Often what happens is when I am distracted by the things of this world, I lose sight of God in my life.
Ask yourself why it is that you want to do these things? Talk to God about it. And if the answer is to grow closer to Him, then it is right and just (and not in vein).
In regards to your friend, first, God sees your friend as child of God. How your friend see herself, does not take away from God’s love for her. There is currently among kids your age. Trans is the current thing. It is such a trend that there are very few who identify as Lesbian these days. My brother’s daughter has identified as a 13 year old boy for the past couple years. She has changed her name twice. Her preferred pronouns is currently him/they (which is confusing to those who are not in and of this culture). I predict my niece will change her name again when she is in her early twenties, because the name she has chose for herself sounds quite childish in my opinion. But, I digress.
You are 17. Our brains are not fully developed until around the age of 25 for boys (biologically speaking). When I was your age, I was attracted to a girl who was two years older the I. She was a little more masculine than other girls. She eventually identified as Lesbian. What I would say to you is that there is nothing wrong with your attraction to this individual. However, I would not encourage you to pursue someone who is still trying to figure out her identity (or is easily influenced by TikTok or current trends).
You can still be a friend to this person. You can even say that you do not agree with her ideology, but that you still care about her (she might get upset though if you do not use her preferred pronouns, which makes it all the harder to be friends with someone who does buy into this type of thinking). For some, this has become a sort of religion for them. In my brother’s case, it has become very much like a cult for them. And like cults, they have cut-themselves off from the rest of the family who do not agree with them on their gender ideology. It is very sad.
Lastly, you can Pray for your friend. God loves her, in spite of what she might think of God. Jason Evert has a portion of his website dedicated to this very topic. I recently heard an interview with the authors of this book that you might find informative. And another resource for Catholics who are struggling with identity, and for family members who love those who are struggling with identity, is Courage.
I have a question for you though before I go. What does science say about this topic? Science and religion are not apposed to one another. Science tries to answer the question of how God created and religion tries to answer the question of why God created. This issue can be address by both science and faith. Maybe we can get into what each says about this topic.