Hi, you have a long and complicated story there, I have a few thoughts but I think I may find it difficult to address everything adequately here. My first thought is that perhaps talking to a therapist about how you are dealing with your sexuality might be a first step. I am not gay but I can understand that it is not an easy life to lead considering how our society handles people who are 'different', whether their skin color, sexual orientation, mental health, whatever. We talk a great talk but obviously the majority of people are not on board with civil rights for all. In saying that, I can see that perhaps watching your older brother living a socially accepted life, married, child, job, house...those are things that you didn't get to grow up with and certainly you have every right to feel you missed out on that. If you feel your sexuality is correct, then growing up alongside your brother would not have made you heterosexual, any more than it would have made you a female.
But I am not a professional person and don't want to cause harm, so my suggestion in that regard is to talk to a psychologist who specializes in sexuality issues.

As for the mood issues, it appears to be situational so I wouldn't be too concerned, except for the fact that it is causing you distress. I think some of us, myself included, tend to live in our heads and we think way too much, connect dots that don't exist, and fantasize a life that probably wouldn't have existed even if circumstances had been different.

Your brother is 6 years older than you. The likelihood of you sharing in his high school adventures is pretty low even had you been full siblings growing up together. I have two sons who are just over 2 years apart and they didn't share experiences either. In fact, they are polar opposites in their personalities. I think the fact that you two are able to connect and love each other as brothers and friends is a pretty astounding thing and you are very lucky that it occurred that way. Perhaps bring yourself more into the present and see things for how they are, rather than how they are not. You grew up loved and cared about, you have another sibling in your life who loves and cares for you, these are good things to think about. And remember, if not for the life you have led, you would not be the young man you are now, smart, ambitious, caring, loving, accepting. Those are qualities you gained from the family that raised you. Try to look at your newfound brother and his family as a welcome addition to your life, another source of acceptance, support and guidance and care that as a young gay man will be a very necessary thing in your life.
I hope this helps some, I don't often write this late in the evening so if I missed anything I apologize. I do wish you the very best, and much success in your future!