Feeling i have been treated wrong

I applied for some training at work. The only 2 things that disqualified you from applying for the training was 1) You must have worked for the company for more than 1 year. 2) You could have no formal warnings on your file. I thought great i can apply. I did. Unfortunately i was not selected to receive the training and so i asked for feedback to determine what i might need to do better next time. I was told i did not qualify for the training because i had in the past sent out some e-mails some of the support staff did not like and it was not fair on them being paid only minimum wage to receive such e-mails. Ok due to my mental health issues i do sometimes get overly frustrated in e-mails i write but at no time have i ever used foul language or anything abusive. Mostly it is asking why something takes so long and why it cant be done now. It is just frustration that i don't write down very well. I have had mental health issues since 2014. At a point in 2015 i was seeing a psychiatrist as i was hearing voices etc. I have since started to try and live more with my problems rather than bother the doctors etc. I do still take medication daily as i have done for over 4 years. My condition just sometimes gets me so angry and frustrated i don't always know what i am writing is wrong in some peoples eyes. I say some people as others have said they see no wrong in my mails. This is the problem i feel after qualifying under their only 2 conditions that i was then refused on the grounds of previous e-mails. Something that was not in their conditions or maybe counted for everyone else. they probably only had to pass the 2 conditions. Not once was i told in the feedback it was due to ability just due to previous e-mails. I know exactly what was said as it was by telephone and my telephone records every call for me. I set this up so i could listen to conversations again sometimes to totally understand what has been said. I cant always remember things well. I am not sure either if i had to to or if i did tell my employers about my condition when i started work. I felt i would never get a job if i did. But not 100% sure if i did tell them. Either way i still feel they have done me wrong and not selected me for training just because i have difficulties sometimes communicating. I have worked for them for over 18 months and never taken a day off sick with my mental health. This is the way they repay me. Anyone got any advice on what to do?