Alright, I have some different thoughts on this subject, so please bear with me for a bit.
You ask for methods of dealing with the thoughts you currently have, but I ask you this: Wouldnt it be even better, if these dark thoughts were simply not allowed to form in the first place?
They come because your view, your "attitude" if you will (I cant think of a better term) is already so very dark. You are telling yourself "I cant do X. It is hopeless. I will lose. I will fail". That's the sort of thinking that leads to these kinds of thoughts. Or, if not these kinds of thoughts, just depression and bad moods in general.
And my advice is the very same advice I always give in this situation: Stop saying "I cant", and start saying "I can".
Think of a famous athlete, for instance. Someone renowned for their incredible performance in their sport. Everyone thinks it's all about skill, but I tell you this: Skill alone is simply not enough to reach that sort of level. They are able to do what they do, really BRING OUT that skill, because they sorta force themselves to go with a positive approach. They say "I can do this". They visualize themselves holding that trophy or whatever. Their absolute focus on the positive can alter the situation overall, and really bring out their potential. And even if they do lose a match, they say to themselves "I lost this one... but I'm going to learn from it, and improve EVEN FURTHER". That sort of thinking, THAT is what allows someone to push themselves to that level. I'm speaking from some experience here (not with sports though).
In this context of course, it's not about you winning some tournament or whatever. But instead, it's about how you're viewing this situation and the "condition", if you will. What if the whole autism thing actually ISNT purely negative? After lots of experience and lots of pondering, I've come up with this opinion of autism: It is not a "disease"... not a purely negative thing in most cases. Rather, it is a DIFFERENCE. A difference that tends to create both negative AND positive effects. And the choice of which you focus on is what determines many things.
For instance, the ease with which you create that art of yours... your mind may simply function in such a way that really allows you to bring this skill out, and that unique function can be an effect of this condition. Or perhaps you have unique ways of thinking and approaching things that could give you an advantage in various situations. You know, stuff like that. The "difference" makes some things harder, but others easier... even if you dont yet realize it.
Something that many find rather unusual about those on the spectrum, is that even if they were presented with a guaranteed cure with no side effects, quite a few would actually REFUSE it. Including myself. Because they've experienced the negatives, sure... but also realized the positives. And realized that it's really just part of who they are... nothing more, nothing less. A difference, not a disease. Dont get me wrong, there are some who absolutely would want the cure, it's certainly up to the individual. But what I'm trying to say is: even though you seem to think it's all bad, all dark, it very well may not be.
But you arent going to be able to find the light in there if you keep going at it with that "I cant" view.
Something I get asked every now and then is why I call myself "Misery", when I tend to seem fairly positive, and am usually trying to help, often saying things like this. It is in fact the name I use absolutely everywhere, so there must be a reason, right? It's because I used to be a VERY negative person. I wasnt ever actually suicidal, but I've been deep into the sort of depression and the sort of moods that you are feeling. I have sort of an inherently dark personality, if that makes sense. And everything always seemed bad all the bloody time. Some very wise people in my life though, they went and showed me that maybe it wasnt... but that my outlook was totally blocking my view. After awhile, I found that they were indeed correct. And once I realized that, I was able to drastically improve in many areas.
And this could very well be the case for you too.
You CAN beat this, you CAN overcome it, and you CAN find the positives, even if they seem deeply hidden right now. But you can only do this if you start telling yourself that you can, and that you WILL. Dont get me wrong, it's not an easy task. But you really, seriously can do this.
And of course, those on this forum will back you up every step of the way. Never be afraid to ask questions on here about anything... helping to learn about being on the spectrum and what it entails, how to deal with challenges, and all that stuff... that's what this place is for. We are here for you, yeah?
Sorry, that was a bit rambly but I really dont do "concise". Hopefully what I'm saying here isnt an incoherent mess, sometimes I have a bit of trouble getting my message through in what I say.