My technique for this is that I teach the dictionary the part of my email that's before the @ symbol. So everytime I need to enter my email address, text prediction shows me my email name after about 2 key presses.
Yeah, you then have to enter @yourDomain.com, but I find this much faster than nothing.
Unfortunately it seems that there isn't any "standard" way to reset the dictionary in Windows 10 Mobile. Hopefully this will be added back in soon. I don't want to try this for fear it will actually work, but you could try adding a different keyboard, removing your default one, then reinstalling it.
UPDATE: A post recently published by Microsoft states that ...
Relax there is nothing wrong with your Windows 8.0/8.1
Your are missing out on a few settings.
To capitalize your "i" go to settings->keyboard->English(your preferred language) and check-on the correct misspelled words on. This will let type much more smoothly.
And once you have checked on you correct misspelled words on. "ill" will be intellisense(auto-...
From my experience (WP8 on Lumia 920), Windows Phone capitalises 'i' to 'I' only on English-US; it just doesn't do it for English-UK. (I have both dictionaries included.)
I can't teach it that I is a word for English-UK; word selection for training only appears to work on words of more than one letter. This is rather annoying, as the work around is to ...
One workaround is to use accented letters where there should be any. For example, replacing the u in F*ck with ū results in a word which can be added to the dictionary and will happily be suggested by the keyboard.