You are doing a great job, don't be fooled by other rl mums' apparently effortless parenting. 
Well done accentuating the positives, praise and encourage DS1.
Actually, do the same for both boys. Start praising them from when they're born. 'Good boy, what a good burp', 'Good baby, aren't you smiley', 'Good job (too American?) DS1! Did you eat all your dinner?' etc etc. You probably do it all the time without consciously thinking about it. It sounds a little phoney at first but fake positivity can brainwash you into feeling you are coping...
When DS1 starts up, take a deep breath and suppress the desire to scream. Say, "Come here {name} what's up?" and give him a hug, sometimes it breaks the cycle.
Wrangling DCs does take it out of you, it's hard for DS1 at his age
to wait, or understand what you mean when you tell him, "In a minute!" While your youngest isn't toddling yet, he can sometimes be set aside to deal with DS1. Otoh when feeding DS2, then DS1 can sit beside or stay within sight, read a book, lisaten to a CD, watch CBeebies.
Every chance you get, get out in the fresh air. Invest in an exercise DVD you can put on at night to de-stress.
As DH is a farmer, are you living somewhere quite isolated? I found it helped having someone just pop in for a chat and brew just to get adult conversation and a small break from the everyday infant stuff.
I know it sounds extravagant but if you don't have Mum or MIL nearby, you could get a cleaner for a couple of hours a week, or even find a local company to take your washing and ironing. That would reduce your workload at home. Anyone with a teen you'd trust to help out?
Ask your HV for help regarding counselling, support groups in your area.
A tip I often see on MN is to look for a parenting course at your local Sure Start centre. So you can pick up new strategies and feel confident.