Once wheel alignment is confirmed and tyre condition/oil leaks onto the tyre are discounted....Stay out of the actual gutter but keep left through the bend (IAM method), try gently locking your right elbow - it'll stop you pulling on the bars by accident (this is with the caveat that your arms are of a suitable length to lock the elbow without putting excessive counter steering into play... I'm a shortarse so it works fine for me).

Work on keeping your centre of attention on the vanishing point of the bend (where the left verge disappears round the corner)... look where you want to go because you tend to go where you're looking. You say you're better when following someone else... I would guess when you're in front you may need to drag your eyeline UP. Look further ahead, it makes you more stable through a corner.

Slow in, neutral throttle to the apex, wind it on as you come through the apex and out of the corner.

An empty car park as suggested elsewhere in the comments is a great place to practice.

All the above advise is good ..
But you need to ask yourself how do I take corners ? Look at your riding style ....type of bike you have ...etc
Do you like most people use 90 per cent upper body only ?
Finish a ride out with your shoulders and arms aching ?
Ok try this on a left hander ....push your right knee into the tank ,at the same time push down on your left foot peg.... the bike will naturally start to turn left ....dropping your left shoulder will also help ....
Time and practice will sort it