Hey Phil,
I've been looking for a site like this and your post was one of the first that I saw. I was also a software developer but got burned out and fell out of it about 10 years ago. Well except for 2 months a couple of years ago trying to get back into it with a local company. It didn't take long to encounter the bad people (and I'm being nice) that always ruin a job and I got out for good.
I think most intelligent people come to a point in their lives when they wonder things like what's it all for, why are we here...or even... what's the point

The stupid people or simple people are lucky on one hand because they can't think past the end of their nose.

I think less intelligent, simple people tend to be happier. When you have the ability and intelligence to do a lot of things or great things you feel compelled, on some level to try to do them. It is frustrating when you don't know what those things are though. So you have to search for them. That's kind of what I am doing now, and what has lead me to a forum like this.
I started out in psychology and did music, electrical engineering, Hollywood and the movie business, programming, learning a number of spoken languages, and some teaching. I still have so many things I want to do or get into that I can't imagine ever being bored. BUT.... I'm finding when you have too much time on your hands and more importantly, when you don't HAVE to do anything, you can get lost, unmotivated and stuck in a rut. That's my issue now.
Some of what you are describing sounds like depression or dysthymia. I have been through both but I discovered how to keep them away. In the movie "City Slickers" with Billy Crystal and Jack Palance, Crystal's character was talking to Curly (Palance) about this exact thing, namely the meaning of life. Curly, an old crusty cowboy held up one finger. Crystal asked "The meaning of life is your finger?" Curly smiles and says, "No, the meaning of life is one thing and you have to figure out what that one thing is"
So like so many things in life, sometimes the answer is so simple that we over look it. Sometimes you just have to stop thinking too deeply about something and just grab the bull by the horns and take action. Just do something regardless if you feel like it or not. There are endless things to get interested in. You can learn a language or languages. I started working on 8 spoken languages at the same time. It was amazing! You can learn a musical instrument, write songs, write stories, or write a book. Find volunteer work to help people, teach something, take classes, get some cool toys like a boat, ski jet, jet pack or learn to fly. I cold go on and on and on. I think you could too. Sit down and pencil some of this out. This will enable you to generate ideas. It might take a while but if you are actively working on ideas and a plan, that in itself can give you some meaning or potential meaning.
I get like you described at times and I realize sometimes it's just because I am tired like if I didn't sleep well the night before. We are more susceptible to depression and things like that when we are tired. When I get like this other times because I let myself get burned out on the things I am doing, I just have to force myself to do something, usually something easy, and then once I get started I am ok and I keep going. Once you accomplish something and finish something or achieve a goal, it feels great!
You are not done! In no way! But if you keep letting yourself think and feel that way then, by the self-fulfilling prophecy, you will be done. I don't know how true the expression "You are what you eat" is. But it is true that you are what you think or what you think about most. So at all cost, get control of your thoughts. Your feelings come from your thoughts and you control your thoughts. Therefore, you control your feelings.

I hope this helps.... I think I feel better now... lol