Hi there, I am sorry to hear about your recent troubles. I was diagnosed with 3 different heart arrythmias when I was 20 years old and I also suffer from anxiety and panic attacks...
Sinus tachycardia is okay. It's any heart rate over 100 that is regular. It happens a lot with anxiety and mist deff panic attacks.
Sinus arrythmia is also okay, as you already know it has to do with your breathing. When you inhale it speeds up and exhale it slows down. It's common in children and younger adults.
The palpations are likely benign and they are frequent in people with anxiety. You aren't expierencing skipped beats or missed beats. They are called premature beats. Either PVCS or PACS. The sinus node is the area of your heart that fires off and controls the pace but both your ventricle and atrium can also fire off (this happens for MANY reasons also mostly benign) when the bear comes from the ventricle it's called a premature ventricle contraction and when it comes from the atrium it's called a premature atrium contraction.
The reason you feel the pause in your pulse is because once one of those areas fire off your sinus node resets and prepares for the next normal heart beat. This allows extra blood to fill the chambers so when the next normal heart beat comes it's often force full to push out all that extra blood. It can feel like a big thud or a flutter. Mine personally feel like a flutter. We don't feel the actual premature beat, what we feel is the next normal one.
PVCS and PACS are benign in a healthy heart as are most arrythmias. They happen for many different reasons and believe it or not they are pretty common. Everyone has had them at some point or another, they just don't feel them. Unfortunately people with anxiety are very in tuned with their body so we feel every little thing. I have thousands of PVCS a day but I have had many years to get use to them so their presence doesn't much bother me anymore.
I would not go to the ER unless you really feel the need to go but I would make an appt with your GP and let him/her know of your new palpations. Have them do an echo (ultrasound) of your heart and put you on a 24 hour holter monitor if they happen daily or a 2 week event monitor if they happen every now and again. I'm not telling you to ask for these things because I think you have a serious heart problem but any new palpations should be evaluated. Your GP is likely going to have to refer you to a cardiologist for these tests.
What you can do in the mean time to lessen them..
Try to manage your stress and anxiety. Stress and anxiety tend to make them worse, as does thinking about them. The more you think about them the more they will happen.
Stay away from caffeine.. it's a stimulate and can make palpations worse.
Don't smoke... Nicotine is also a stimulate..
Avoid drinking alcohol
Get plenty of rest. I notice when I don't get enough sleep I get a bad bout.
Have your potassium tested. Sometimes having even slightly low potassium can cause palpations.
I eat 1 banana every day to keep mine good.
Magnesium is a good supplement.. It relaxes the muscles. It won't cause them to happen less but it can help you not to feel them so much.
If after you see the cardiologist and they are bothering you greatly you can ask for a low dose beta blocker to take as needed, like when they get really bad..
I hope this helps