For starters, you might consider getting help to file these back taxes. It will cost. But I’d recommend it.
If you do them yourself, with or without Turbotax, here are some considerations:
1) You cannot, ever, mix income from different tax years on one tax return. You must file each return separately, year by year. For example, your 2014 return must ONLY report income from 2014 slips.
2) if you are using tax software such as TT, you MUST use the version of software only for that tax year, since credits and base amounts are changed each year,as well as changes to the ITA.
3) Prepare each year 2014-2018 ( 5 different returns) separately. If you mail them in, send them alll together. If you are able to file electronically, send earliest first.