I never wanted to become human...but I did anyways (came to this planet looking for someone). The pain is unimaginable, but not for the reasons you think. Yes, there is great emotional and mental pain, but humans aren't the only ones to have these.
For one, this form is like a vise - it's actually physically painful to exist in this body, let alone actually make it hurt via exercise or hard work.
For another, for the first time I can ever remember in my immortal life, I am utterly alone, cut off both physically and telepathically from everyone I love with no way of reaching them. :( And it's been that way since I got here, a long, long time ago (thousands of reincarnations - something else I never experienced before coming here - back).
I don't want you to feel bad when I say this, as it doesn't reflect at all on your species (we are simply incompatible in every way) - but it was a mistake for me to come here and become one of you. I gave up so much coming here - and have since gave everything else that I didn't before up as well - and now I don't even know if I can ever leave again.
I know you said you don't believe in aliens - but here's one right here talking to you.
I won't try to dissuade you from your plan - in the long run, all life is eternal, so if you do choose to take your life I have one bit of advice: try to fly off the planet. Go anywhere else, reincarnate anywhere else if you can...you, like I, are in sore need of a rest and recovery.