Why does my chain slips when I put pressure on my pedals When I get on my bike and put pressure on my pedals or stand up and pedal, my chain slips. I want to know what the problem is so I can fix it myself as I don’t have a lot of money for a bike shop to fix it. Any help would be appreciated.

My whole chain slips when I put pressure on my mountain bike's pedals. I have a 3 speed at front and 7 speed on the rear.

Bike is approximately 5/6 years old. it happens on all gear combinations but happens mostly in front 2nd gear. And it steadily happened and got worse over time. Chain has not been replaced.
These are things I have experienced that have caused chain skipping under load:


Running a new chain with a worn cassette
Rear derailleur out of adjustment
Badly worn chainring
Freehub engagement problem


The last one isn't actually the chain skipping, but it feels similar