I think rejection might be one of my triggers for self-injury

Don't cut! You're so much better than that!
Rejection is something that many people fear.
That's kinda how I feel when I show my drawings,
or even ask the person if they're really my friend.
We don't like the feeling of rejection because all
or us, whether we admit it or not, seek approval
from those we care about.
Trust me, these feelings CAN be stored away.
Just remember this phrase:
"I ain't here to meet your standards".

And please... don't cut yourself.
Inflicting pain on yourself won't get anything accomplished.

 

It looks like there's a few problems going on here. 1) your writing group, it sounds like it isn't really a safe place for you and that's upsetting you; 2) feeling disgusted by your sexuality; 3) worrying about your mother finding out; and 4) self-harm.

The two I feel most qualified to address are two and four, so I'll talk about those and leave other ECers to work on the rest. First, it's not immoral or bad to be uncomfortable with or even disgusted by your sexuality. These are emotional reactions, over which you have no control. Emotions are your mind's way of explaining to you the input it's getting from your five external senses and your sixth internal one (i.e. your thoughts). The senses can lie and thoughts can lie, but emotions cannot. That doesn't mean emotions are objective by any means. What your emotions are telling you is that you haven't fully accepted your sexuality, nothing more, nothing less. And that's okay.

Accepting being gay is something that comes slowly to a lot of queer people, a quick perusal of the threads here in the Support and Advice section should make that clear. A lot of us have significant cultural and familial barriers to acceptance, and we internalize these messages. Working out what your barriers are will help you to understand why you're having this difficulty, and from there to working on it.

Self-harm. I've done it, and so have many of my friends. Self-harm is a coping mechanism for dealing with emotional stress. By triggering the senses sharply, it draws me out of my mind and into my body. It reminds me that I am alive if I feel dead inside. Stopping self-harm, therefore, is best accomplished by finding other, healthier ways of pulling you out of your mind and into your body. This process is called "grounding."

There are a number of grounding techniques. They range from running cold water over your hands to eating food slowly and deliberately, focusing on nothing but the feel and taste of the food in your mouth to exercise to meditation. If you want, I can go over grounding exercises with you in more detail.

 

A few grounding exercises

Eating an orange. There is nothing like an orange. It has a unique color, smell, texture, taste, and smell. More, all of these are striking to the senses. Slowly peeling and eating an orange is an excellent grounding exercise. The point is to fully engage in the process. Savor the smell, the feel of the peel in your hands, the weight of the orange, inhale deeply as you open it to get the full citrus scent.

Meditation. There are a number of these, some are harder than others. The first mediation I ever learned was to lie on my back, slowly breathing in and out through my stomach. Then, clench the muscles of my feet for a few breaths and unclench them. Then clench my feet and my calves for a few breaths and unclench them. And so on up my body until I was clenching my whole body.

Running cold water over your hands. Pretty much exactly what it says on the tin, provides a mild sensation to the skin. In that same vein...

Ice. Hold an ice cube to your skin. This, like cutting, is painful, but unlike cutting it won't hurt you unless you hold the ice cube to the same spot on your skin for a long time.

Petting an animal. If you have a pet, giving it some affection, savoring the feel of the fur, can be an excellent grounding exercise.

There are hundreds of them, that's just a brief list.