Alright, I'll share a few of my tricks, and leave off the advice for once.
We have a "crying corner" in the house. Whenever the tantrums start, they go to the corner. They can scream and cry and whine as much as they want while they're in the corner, but they can only come out when the tantrum stops. They make the decision on their own when they're ready to calm down. Once they calm down, then we can talk about why they were upset and how we can fix that. I haven't had to use this on my oldest in a LONG time, but my 3 year old is there quite often.
Another trick of ours...anytime the girls go shopping with us, they're allowed 1 small treat. But they only get this treat if they behave while we're shopping. If they behave badly, the treat goes back on the shelf. Now, call it bribery of you like, but it works.
Also, realize how BORING the grocery store is for kids. Involve them in the grocery shopping, let them help find the things you need. If they feel like they're helping you, they're usually happy as can be. Or talk to them, tell them stories, sing songs with them...anything to keep them entertained.
That being said, I have had maybe 3 or 4 instances of tantrums in the grocery store in my girls' lives. They just don't do it...and if they do, it's usually because they're tired or sick.
Don't set your kids up to fail...plan your grocery shopping around their schedule so you're not taking a 2 year old into the store right at his nap time or bed time. Plan trips in the morning or in the afternoon after naptime. And if a lollipop will keep your child happy and calm so you can get your shopping done, then give him one.
As for WHY they throw tantrums...well, after reading so much about your marital issues, you MUST realize that you're child is affected by that tremendously. It's so stressful for children who have no concept of relationships to see their parents fight and argue. Even if it's unspoken, don't you think he's picking up on the tension between the two of you? He doesn't have the words to express how that makes him feel...scared, unsafe, etc. Put yourself in his 2 year old shoes and imagine how you might feel.
Okay, so there's a little bit of advice there, I guess.
Love, Heather