Job resignation rejected. Advice needed.

I am a fresh grad working for a smaller engineering firm. With no specific skills relating to my work responsibilities in the Oil and Gas industry, i find myself struggling in every bit of task to get the project done on time and on budget. I am working with a project manager. I have huge problem mixing around with my coullegues. As I am new to the job, i need a lot of assistance to get my work done. Since a lot of talks behind my back i find it even harder to communicate with people to obtain information. I am very quiet and would just like to get things done and rarely would like to start any conversation with people at all. I guess it also due to the fact that I find it hard to cope with work therefore i find the need to spend lots of time on the screen to understand my tasks. Talks behind my back saying I am not being productive, not worth being trainned since i would probably leave within 2 years, pissing around all day long gets into my nerves and spoils my motivation. I had a chat with my boss and he rejected my resignation, stating that its all due my negative thinking and that i should ignore all the talks behind me. He is really nice person and motivates me to continue. Its like trust being put on my shoulders which I am very grateful. But i feel that i am being really helpless in this situation and do not know how to make things work for me and him. Advise needed. Thanks in advance.