Going flat out during each session will not effectively build tri fitness.
Whole books are written on the subject and coaches spend years learning their craft. So tap into that expertise
Best book I've come across that is suitable for newbies is 'Starting Out Triathlon' by Paul Huddle & Roch Frey. Available from Amazon from £3 secondhand
Hope this helps
if you've not been cycling long then give yourself a break - if others have been riding longer then will have become faster.
keep going - read some of the books and try a few things out.i'm sure you'll get faster!