If I'm 40, is it too late for me to develop a portfolio of skills from scratch that can eventually earn me $10k a month?

No, but you'll have to change your mindset. You write like you need to acquire a whole new skill set of some sort so that you can eventually make a decent living.

The truth is you already have the skill set you need. I am sure there is something that comes ridiculously easy to you. I'll take knife sharpening as an example.

Let's say you can put a razor's edge on a dinner knife without leaving any stray marks on the blade. People want that. They live years with dull knives because they don't want to scratch up their good silver. You teach people how to get a sharp blade, without stray marks, then you do affiliate marketing for the system you use, then you sell them awesome knives. Maybe you branch off into a fine cutlery or silverware business.

The point is that you already know how to do things others don't. Figure out which ones excite you and that you'd love to share your knowledge about.

There is one on-line marketer (Russel Brunson) who started by showing people how to make potato guns.

There are people who do fivver 99design arbitrage, and make a good living at it.

It's just a question of being creative with what you already know. I think a great place to start would be at Altucher Confidential - Ideas for a World Out of Balance. He's got some great advice for keeping you at the top of your game and creating the life you want.