Finally contacted my GP but now I feel stupid...

Hello, I'm new here and this is the first time I've ever reached out to other people. 

I'm 31 and have been suffering with chronic depression for over 5 years, well...only those 5 years have been treated. 

My entire life has been a struggle. From being forcibly removed from a Wendy House as a child, screaming because it was safe and I was left alone to getting fired from every job I get because I "don't fit in" or get on with other people there eg. I just don't talk to them or I don't have the same energy.

I've never had friends and in my adult life I count 1 and the rest are people online. The distance from them makes me feel safe. 

I play a lot of online video games. Not loud, shooty types but one's like Final Fantasy 14 when I can just exist in a world for a while and gather some stuff to craft with for hours. I don't have to talk to anyone but I like seeing people there. 

I'm also an artist but I never really finish any of my pieces ha. 

So today I finally had a requested phone call from my GP and...i froze. I panicked and I said all the wrong things and I just couldn't express why I feel I may be autistic. I had the conversation all planned out on my head but when it came to it I just crumbled. I felt stupid and attention seeking...everything I said could be put down to depression or just...nothing. I'm so upset but I couldn't explain my life in 10 minutes. 

The good news is, is that he told me to send him a written letter via post explaining why I wish to be tested etc. But I'm struggling. It's not that I don't have reasons and examples and family members to back me up, it's that I feel I'm once again just going to be washed over with the "you're just eccentric" or "it's the depression" brush.

I just...can't say what I want to and this phone call took years for me to make and I feel just so utterly stupid. 

Is there anyone who could give me a few tips to help filter my thoughts to put them on paper for me to send to him? Anything that could help properly express the struggles I had growing up and in adult life? I feel like because I cant express them, I can't make anyone believe me and that is devastating...

Thank you for your time :)