Really don't know what to do, how to feel better.

Sometimes, I feel sad/ depress. It come and goes. But, these two weeks, I just can't get over it, feel terrible all the time.
I feel tightness/pain on my chest, body arms and legs become cold, when I think of something that bothers me. I can't concentrate, can't work, can't help but feel great sadness, and tightness on my chest, feel my head going to exploded.
I am seeing a guy for 9, 10 months. we have great time, and great connection. I feel that he has been withdraw/ distance for last 5 months.... I though he was busy or going through something, but, just realize...maybe not... I could/ and would like to write about what happen. But,... I know it's not about him, it's about me.

I tried to work on myself, and read some articles, and I understand that I have to have a happy and fulfill life first, then everything will come along. I think that's the problem. I don't have happy and fulfill life. I work at home by myself, I am not too social, I have few close friends who all married....
I saw this guy again a week ago, and feel that something changed. Just the feeling of he might not be that interested anymore, and the problem is me. If I am not happy, then I am not going to attract any right person, and have the relationship I want.
I know what I should do, I should focus on making my life better. But, I feel so sad, can't sleep, can't eat, can't work. And, this tightness on my chest, just make me feel trap and going to exploded. I can't work, that make me feel more trap, and sad, then it effect how I think about myself, and the relationship, then I can't feel good about myself, and the relationship (with anybody, not just that guy) will become worse, then I feel more sad, can't work, can't do anything, can't even walk on the street without feeling want to put my head in the sand.... It's like a cycle, it's like I just sink in more and more, don't know how to get out, and what to do, just feel going to exploded. don't really know how to describe it.
BTW, I do creative design work, but I have deadline to meet. now, I am so behind, don't know, and can't see myself how to finish the projects. I can't get anything/ idea out of my head, can't get myself together to work, feel like I lose all my ability, and know how to do anything...
Please help!! appreciate!!! I know it's long post, thank you for reading.

You might trying seeing a therapist they would have all the tools needed to help you learn how to manage these thoughts and help you get a schedule to stick with that will make it easier for you.

I can't get anything/ idea out of my head, can't get myself together to work, feel like I lose all my ability, and know how to do anything... - Everyone has the occasional crisis I think, is there anyone you can talk it over with, can you negotiate more time? Perhaps if you can do this you will feel better about it and some of your creative energy will return? I certainly find that nursing these worries does not work well.

It's been days, I sit in front my desk. feel un-describable sadness, tightness. keep thinking about the guy I am dating, might not be interested, or it will not work, because I am not a happy person, can't see myself finish the projects, how am i going to find anyone, if I don't even feel good about myself, don't know what do do with my life......, just keep worrying, can't get it to stop, then feel exhausted, or going crazy. I can't stop thinking, and start working.

This is awful. It sounds like you are having a full blown crisis. All things pass but I would guess you are rather vulnerable right now and help might be required.

Are you getting help? Have you seen your doctor?

There is no reason to suffer on your own.