Baby won't sleep!!!!

My baby will be 7 months old this week and he still doesn't sleep very long at night. I feel I have tried everything but he won't sleep longer than 3 hours at a time, 4 if I'm lucky! I'm literally crying as I write this post because I feel like a bad mom. He's been screaming for the past 45 minutes and I've changed him, fed him, gave him gas meds and it still wasn't working. I swear the older he gets the worse he keeps getting as far as sleep. He is capable of soothing himself but he won't always do it. Once every few weeks he will throw me a random night where he sleeps 9 hours straight but of course I tossed and turned so I didn't sleep. I can't figure it out. He has a white noise machine, he's nice and warm, fed, consistent bed time routine, eating solids, lots of tummy time and almost crawling.