I don't understand how you've had money in a S&S ISA for the last 20 years that hasn't grown very much. Are you sure?
Who is the provider and what are their charges? That could be the issue.
If you choose a low cost provider and properly diversify your investment then you should get a decent, higher than cash, return over time.
However if you've not done this, the charges could have significantly affected your return, but it could well be the wrong time to try and change it, because if the funds you have invested in are down, then for any money you take out of them, you crystallise the losses, whereas if you leave it in there, it could go back up.
But I'd advise against picking your own shares if you're not knowledgeable and don't want to be an active investor. You might get lucky and pick a winner, but equally, you could lose part of your investment, or at the very least, not see much growth.