How to be hair

Hello there.
First of all, remember that you do have friends and that a few people who care deeply for you are far important than many people who you just know superficially, people who seem to have great social lives still probably have the same number of close friends as you do, so don't worry about comparing yourself.
Regarding the kickboxing, if you want to continue the sport with the people you know there, could you switch to another coach? I don't know how your organisation works, but maybe that's a solution. You could talk to your current coach about your concerns with time and anything else. It's a really positive thing to be involved in sport and I would not advise you to keep going with it.

If you want to get to know new people, maybe join a new club/activity? You seem stretched for time with your kickboxing, but maybe there is something within school/sixth form/college that you could try? Volunteering is another way to meet people.

To look after yourself, it might be beneficial to start a journal if you haven't already. Externalising your thoughts by writing/typing them can make the world make a lot more sense. It might also help in working out how to feel more confident in social interactions. You could set yourself little targets, like "make someone I don't know smile today", or write down the things that made you feel awkward or embarrassed so you can identify how to make the outcome better next time.

I really hope this helps in some way. Good luck!