It's really difficult to offer any advice because it is not possible to know what system or style you have learned to drive with.
Sometimes going back to easy basics and doing them well in fairly quiet situations can rebuild your confidence? Then as you go along and are driving along talk to yourself and tell yourself what you see. Then ask yourself what you need to do? Slow down or stop? Move position from where you currently are left toward the kerb or right toward the centre line?
Take each hazard and each obstacle bit by bit. If information overwhelms you, slow down. It sounds really obvious, but this is so crucial in a busy unfamiliar situation where you are trying to see what is on signs, route boards etc as well as looking behind you and alongside you. Once you have missed information boards, missed junctions etc its easy for the panic to set in. A Sat Nav can help with a soothing voice? Busy roads will test your skills to the limit (particularly if the skills you have practised are not fully embedded and routine) as the additional road users demand extra concentration and quicker decision making.
So for every anxious driver there are probably another 50 behind you all in a stressed or anxious state. You are not alone. The moment you get a natural chance to stop (traffic lights; queues etc) take time to breathe in and then breathe out in a long over exaggerated exhale. Do this two or three times then tell yourself you are doing ok. No one is perfect but knowing who is alongside you and behind you at all times when you need to move in the road is a big help, then other people won't take you by surprise. Good luck.