I understand. I underinvested in myself for years. It got to the point where if I had to dress up one day, I didn’t have anything nice to wear if I had to dress up again the next day.
In your shoes, I would start by going through what you’ve got already. Anything that you don’t like or doesn’t fit (or you’re unlikely to fit into any time soon) - get rid.
Identify the gaps in your wardrobe so you can shop purposefully. Do you need more jeans or summer dresses? Do you need a certain item of clothing to make that nice skirt work?
Have a read through of this thread: https://www.mumsnet.com/talk/style_and_beauty/5091601-what-little-updates-are-you-adopting-to-stay-current?page=2&reply=135843101. Might give you some inspiration and ideas on how to update your look.
Maybe get your hair done, eyebrows and nails? That can help make you feel nicer.
I’ve heard good things about John Lewis personal shopper service.
There’s a Facebook group - Seasonal Color Analysis. If you post photos, they will work out what season you are, so you can pick clothes that suit your colouring. Or you can go for a paid service like House of Colour.
Have another think about how your shape and have a google on how to flatter it. Reason I say ‘another’ think is because your shape may have changed a bit following having children. Also, it’s possible that there are some ideas you haven’t tried. For example, I’m a classic pear shape. I’ve always avoided short jackets. I’ve now realised the error of my ways and have found some items that look nice. I’ve also realised that tops that end on my hips aren’t too flattering.
Have a look through your jewellery and make up. See if there are any ‘gaps’ in your jewellery or if any items of clothing would benefit from a certain type of jewellery. Think also about belts and bags. Perhaps photograph yourself in some favourite complete outfits. There’s some good guidance online about ‘styling’ an outfit. Perhaps go to a make up counter for a fresh make up look.
I’m actually not stylish at all, but I’ve been where you are now and I’m slowly looking a little better.
If all my ideas are rubbish, at least I’ve given this thread a bump.
Good luck.