Yup. I have those feelings now and then. I too have a son who is 15 and I often feel guilty when I get those thoughts.
Here's what I did. I changed the way I perceived those thoughts. I did not try to get rid of them, just change what they meant to me. Now when I have those, "What's it all for?" thoughts, my immediate response is, "It's for whatever I want it to be"
Sobriety has given me a blank canvas on which I can paint whatever picture I want my life to be. My old life drinking had no meaning except when to drink again. I am still unsure what meaning I have in my sober life, but what I do know is that I can make it mean whatever I want it to. I don't have all the answers right now. I'm not sure what picture I want to paint. I just know that I have an abundance of choices.