I'm not a huge fan of motorways either but the more I have driven on them, the less worried I feel.
I would recommend driving on them as often as possible. The best way to overcome your fears is to face them. The more you drive on motorways the more comfortable and less scared you will feel.
As bexsimpson says, when you're joining a motorway, 99% of the time people will move over for you, and if it's busy, they will slow down and let you in a gap. You rarely see anyone stranded on a slip road...although I once did have a scary experience when I was coming down a slip road and the person refused to move over and I had to break quite hard...but that was their stupid fault, not mine lol.
My biggest pet hate are those huge, multi-lane roundabouts on motorways...it's really hard getting in the right lane, and switching lanes, when you've got about a million mad drivers whizzing past you.
Anyway, my motorway nerves got much better after just forcing myself to drive on them lots of times. Something else that helps is having a sat nav, because that takes away the added worry of trying to work out where you're going and following signs, and your brain is more free to concentrate on the road and on what other drivers are doing...
good luck