I promise you everything is going to be ok.
I was devastated when I was diagnosed but I very quickly came for terms with it. The diet is easy to follow once you get your head around it, and GD is a totally manageable condition. I ended up taking 2 metformin per day to control my fasting levels. I felt totally well the entire time.
I had regular growth scans which was so lovely - so many extra opportunities to see my baby and be reassured that all was well.
My baby’s growth slowed a little towards the end so I was induced at 38+3. Labour didn’t really progress and ultimately I had an emergency c-section but I felt totally supported, informed and cared for the whole time and even though it wasn’t what I wanted I had a really positive birth experience. My baby was 8lb at birth, and was healthy and well from the start.
You have absolutely not failed your baby. You couldn’t have prevented this, and it isn’t caused by anything you did. It’s a hormonal condition. It will not cause your baby harm, because you’re a good mum who will follow the advice and manage the condition, whether that takes diet or medication. Please be reassured that everything will be ok 
I hugely recommended the Gestational Diabetes UK website and Facebook group. It’s a hugely supportive environment and will give you so much access to support and advice. The dietary advice in particular is excellent and often much better and more up to date than NHS dietary advice for gestational diabetes.