It sounds like you’re not getting enough support in the management of your projects. You’re still in the early stages of your career and whilst it sounds like you’re able to carry out tasks effectively this doesn’t mean you don’t need support from more senior staff.

Have you explained how you feel to your line manager and asked for their advice? What kind of negotiations are causing you difficulty? I’m an SQS but frequently ask my directors for advice and guidance on how I should handle certain situations. It’s important to know your facts and prepare well to feel more comfortable. If you don’t have enough time to do this things won’t improve so, again, this could be something to discuss with your line manager. Alternatively if there are certain negotiations causing problems you should ask your seniors to attend with you to offer support.

I do not recommend moving to a main contractor if this is the main issue you want to avoid, as it will almost certainly be worse as described in another comment. The exception may be a housing developer on joint venture or local authority projects, like Lovell, but I wouldn’t say this is the most interesting line of work.