I'm not sure if this will help OP, but just to say I felt exactly the same as you a year ago. Found a degree course I fell in love with, but I didn't do great at school, flunked out of college, so I was still unsure of myself.
I decided to do an access course and it has been life changing! A lot of it was due to having access to an amazing tutor, (not sure you have that privaledge with OU) who really brought the best out in me, instilled me with confidence and helped me to realise my potential. I was gobsmacked to leave with a straight set of distinctions. As it had been 20 years since I'd been in college, I got the chance to practise writing again and get into the studying mindset. But also, the access course is so geared up to preparing you for uni, they help you fill out the form, they help you with your personal statement etc. They support your application so that it's the best it could ever be.
I'm starting uni in a couple of weeks, and although excited, I don't feel overwhelmed and I am confident I can manage it.
What I got most from the access course it confidence, so this is all it really takes, you sound like you have the passion and ability, all your lacking is a little confidence.
And yes, universities do love a mature student! I think you'll probably be surprised at how much you've learnt just through adult life and being a parent - it all comes in very useful, I promise. I was the oldest in my access course - and the most focused. All the younger students had hectic social lives that interfered with studying (absent with hangovers etc). and often had big dramas going on in their lives which distracted them from college, whereas we learn to cope and multi task better as adults and parents.
Definitely call the uni, tell them you really want to study there and ask for their help. They will love it! What they want most, is an eager student.
Good luck!