How can I retrieve the individual amounts of my different pensions for prior tax returns, since the returns only have the total of all pensions?

You can try to get your tax account transcripts, particular your wage and income transcripts.  That should list each 1099-R individually.  Transcripts should go back 10 years. 

https://www.irs.gov/individuals/get-transcript

If your 1099-Rs are not in your transcripts, then there is no other way to get the information from the IRS or Turbotax.  You would have to contact the individual payers.