If you've only been diabetic 5 years, you've always previously had good control and in target hba1c and you have no other diabetic complications (neuropathy, retinopathy) gastrparisis wouldn't be my first thought.
It could just be that you're 5 years in so, no more honeymoon & things are just a bit more challenging. It could be other hormones going on.
One thing that did strike me from your post was the comment that 17 units of lantus had always worked fine. It's just, that seems like quite a static, fixed approach to diabetic management, which is fine when it's working. But unfortunately diabetes is a condition where sometimes things change, occasionally for no apparent reason, and what worked yesterday just doesn't want to work today. So sometimes there is a need for the management to be flexible too. Requirements for background insulin can change - if you are going up overnight you might need to increase your dose of lantus, or think about splitting it so you have less in the day to avoid the daytime drops, or think about moving to a different basal insulin as lantus is not actually that flexible (changes take 2-3 days to take effect) and could be said to have some instability issues.
Maybe have a read up on basal testing -
https://mysugr.com/basal-rate-testing/ - and have a chat to your nurse about next steps, especially if you have had concerning hypos.
& don't be so hard on yourself, nothing's slipping away