Hi, I'll talk about my experiences, maybe it will give you some help ;).
I'm an average guy, I was never fat, my body have the tendency of being slim, I know a lot of people would love to have a body that can get slim easilly, but there's the catch I like bears, so my concept of beauty is different from the common and specially from what I look, so I always had insecurities about myself and my body. For some time I though I had to put on some weight and try to reach my "beauty pattern", but you know my body doesn't allow that. But with time I realized that's a stupid way of thinking, I found out that there are people that found me attractive, and that made me feel better about myself, better about my body, and made me accept more who I am.
I was 23 when I kissed/had sex for the first time, I believe for gay men is common to have their first experiences after their 20's I think mostly because of insecurities about your own sexuality, being afraid to come out. One day I was coming back home on the train, and I saw a guy looking at me (I don't really know if he was actually looking at me, but still, hehe), and that was the day I realized that I couldn't stand it anymore, I couldn't stand being afraid of my sexuality, of meeting someone, of coming out, everything. I was really sad, I don't know how many times I cried on the shower because I was alone, because I though nobody would ever like me. I know what you are going through, because it also happened to me, and happens to most people.
So that day I decided, I created an account on a dating app, and from there on things changed. I started receiving messages from other people, other people who found me attractive, and that made me feel good. From there on I had my first sex experience, my second one and many more, and ended up finding a boyfriend at the end of all that :cheers:.
My point is, those insecurities are common, but because of them we are afraid to go after what we want, and instead of trying we let the insecurities take over and create things that aren't true, and make us unhappy. You have to overcome them and go for it, go for what you really want.
And in my experience, yes people care about appearance, it's important, but it's not as bad as you think it is, people think you have to have a model body to be liked to be accepted, and in my experience it's not like that, really, people have different tastes, different opinions, so you'll always find someone who finds you attractive. Now about cock size, although we hear a lot, but really size doesn't matter that much, unless it's really really small, you'll have no problems with it, and at least in my opinion I prefer a medium sized that a huge one. It depends on the person there are people who are size queens, but again, it's your insecurities talking again.
I don't know if I could express myself the bet way possible, hehe. But my message is, we create a lot of conceptions in our head because of our insecurities, we hear a lot of stuff that scares us, but in the end they aren't true. Don't be afraid of being yourself, of loving your body even if it isn't your concept of beauty, of looking for someone. I wish you all the best and I hope I could help you somehow :D.