So I literally JUST came out to my grandma

Here to listen I am sorry to hear your grandma reacted that way and was not more supportive. I know it took a lot to tell her. It's unfortunate that many people still believe that sexual orientation is a choice but as I am sure you know it certainly is not something we choose.
Is she the first person you have told?

 

I realise this won't change much, but try and think less about that fact that it went badly and concentrate on the fact that you were brave enough to get it done!

 

I also wanted to touch on the cutting aspect if you are comfortable talking about it hun. I speak as someone who cut for years and years starting when I was 14. Have you been cutting long...? I know you feel panicky because it went badly and probably feel like you will explode if you don't do something (cutting) ...do you have any other coping mechanisms you could try right now to help you feel less like cutting? (Going for a walk, talking to a friend, a hobby etc) ?

 

I know it is so hard and as silly as it may sound, because this is a personal issue, try not to take it too personally from your grandma. Her lack of understanding is her loss, her ignorance and her problem, I understand it hurts not to have her support you however also try to understand that how ever much she may not agree, she is a different generation many of whom will never understand. I sure as hell know I'd struggle to tell my grandma if the time came.
You have been extremely, extremely strong and confident to tell her at all. Please try to focus on that now, when you feel as though you need to cut, remember how strong you have been to tell her, and how strong you can still be.

 

Of course it is hard. But she is not forcing you out or anything, is she?
If you have summoned the courage to tell her knowing she may not react well, then you are strong enough brush off her negative response.
If she's said "when did you decide to be gay", is it possible at all that you could try to educate her? Explain to her that this is not a phase, or a decision you made one morning, you're still the same person you were before you told her - the only thing that has changed is her knowledge of you, not you as a person.
It's not impossible to teach an old dog new tricks. (Please note I am not calling your grandmother a dog, simply a metaphor!)

 

All you can do is do the best you can, and be yourself. It's natural to doubt when the people around you are telling you 'it's wrong' and 'when did you decide this' and 'are you sure'.
But as I said before, you were strong to tell her, do your best to stay strong, and you're not alone in this.

 

Some people take time to get used to the idea of a relative being gay. Give her some time, she's probably just shocked. Some people take a few weeks or months until they come to terms with who you really are. Show her that you are still her grandchild and that you are the same person you were before you came out of the closet.

 

It does take some people (usually relatives it seems) a while to accept when someone comes out. Perhaps it is a shock and they need time to digest it all. It was super brave of you to tell her at all and I agree with previous posts that you should focus on how strong and brave you have been, just keep being who you are and give grandma some time. who knows she might one day come around. hope your best friend was more supportive of you coming out? And please do not let a negative reaction discourage you from being who you are or coming out to loved ones in the future. If someone reacts badly that reflects on them, not you!!

I do want to stress you are not alone in the cutting, it is a coping mechanism and normally a symptom of anxiety/depression. Have you found you have triggers that make you feel like you need to cut?? Do you suffer from anxiety at all by chance? Poetry is a great outlet for emotions and thoughts, as is a journal. Other coping skills for cutting can include relaxation breathing, getting out of your current environment if you can such as a walk (hard if your grandma will not allow it) hobbies, talking with a friend...also music is another great distraction to help calm you down when you feel like cutting.
What worked for me finally was a replacement 'pain' it sounds odd but cutting releases endorphins in your brain that make you feel more relaxed and even happier due to the minimal pain it causes. You could try something like an elastic on your wrist and snapping it instead of cutting or my personal one which is holding an ice cube in your hand until it hurts too much to hold any longer and deep breathing while holding it.
I know how hard it can be to battle cutting and the conflicting emotions that go with it, it is 'normal' and 'common' but also not a healthy activity. You may feel worse about it later on due to scars (I only wear pants now in my 20's due to cutting scars and wish I didn't have them) If you can talk to a counsellor I highly reccomend it but I know that is not always easy as a teen (been there) so if you cannot then please try out some other coping skills and replacement pain methods I listed above (ones that are not harmful to yourself of course)

Hang in there and keep being yourself. You are doing great knowing who you are at a young age and being brave enough to speak up about it xx

 

you are you, you come as one package, keep being you and dont be discouraged!