I have a small house too and I find I get very stressed and tend to crack it if the mess/clutter gets too much. I've been following Flylady for a few years now. Not religiously - I don't shine my sink - but the rules like:
You can do anything for 15 minutes. It doesn't have to be finished, just set a timer for 15 minutes and go go go until it goes off and then STOP. Do this once a day, once a week, several times during a day, whatever suits you. It will be done quicker than you think.
Start small. If the cupboard is overwhelming - start with the easiest shelf. If the room is overwhelming, work your way around clockwise (or pick the easiest spot). Do one drawer. One box. Set the timer for 15 and just go for it.
It doesn't have to be done all at once. It didn't get that way in a day and it won't a day to clear it out. It is an ongoing process. Sometimes I find I'll clear a space then come back to it a few months later and find I can remove a whole heap more.

I have joined a decluttering group on FB. "70 Day Christmas Decluttering Challenge". It's day 29 but it's not too late to join if you want. The group leader posts a daily challenge. People post pics of before & after if they want. Everyone is very supportive, helpful and encouraging. There is a list of the challenges someone on there so you could catch up or just work through slowly. I've only done about a quarter of them but it has made a huge difference already.

My rules are:
Clothes - not worn for 12 months - gone. BUT I assess each piece as it goes so I'm not completely strict with that. I'm struggling with the idea of getting rid of my wedding dress. I recently donated all my work business shirts. I haven't worn them since I was pg with DS and my stomach is just not shrinking back so they just don't fit. I kept them thinking "I'll lose the weight and they'll fit again" but realistically, even if I did lose the weight, I still wouldn't wear them because I HATE ironing and very rarely do it.

Toys - If they are past the age for it, it goes. I struggle with toys that were gifts. I recently returned a whole heap of FP stuff to MIL as I felt I couldn't sell it (she is quite attached to the FP brand and kept toys from when her kids were babies ) so I returned them to her to pass on to her daughter if she ever has kids. At least they are out of our house.

Tupperware - if not being used ever - out it goes. I really struggle with this one too as it's soooooo expensive and you know, it could be useful. I culled a bit a couple of years ago, and I have removed a few more in the last few weeks.

Ask yourself - is it useful? is it beautiful? do you love it? If the answers are NO NO NO - out it goes.

One thing I like to consider to help me make a decision is this - if the house was to burn to the ground, would I miss this? Would I want to replace it? Could it be replaced? Thinking along those lines, there is little I would keep.

I find gifts & sentimental items the hardest. I scan or photograph kids artwork and only keep certain special items like Mother's day & Father's Day gifts. I have a box for each child. That's it.

Good luck.