Hi Robyn and welcome to the forum.
I've had it for 28 years and I can still get it monumentally wrong. That's where a support forum, such as this, comes in - you can compare experiences with others as well as getting links to lots of useful info.
Testing and more testing is the key, provided that you examine the results logically and (perhaps more importantly) honestly. Having said that, don't be too dismayed when there is no apparent explanation for an unexpected result: sometimes, D is just like that!
(apologies to the T2's who struggle to get testing stuff - my sympathies are with you on this)