Good morning again, and welcome again, Juju. You're gaining on it, so stick with it!
I found, when first getting off the coffin nails, I really needed the hand-mouth motion in addition to the nicotine replacement. In my case, that's been a harder part of the addiction to kick than the nic. I'm way down on nic, but I still need the pacifier after a year and a half. Regarding your questions, 18mg is probably not too strong for a starter juice. That's where I started, and I was 1.5-2 packs of Reds a day myself. Needing the nicotine as well as the constant struggle with the demon, you're probably vaping a lot more than you will be in just a week or two. It's possible, at 18% and frequent chain vaping, you'll be getting too much nic compared to what your body is used to. I know this sounds impossible
-x) but it happened to me. In my case the symptoms were the dull headache above the eyes; but others are diminished appetite and mild nausea and shaky hands. It is also possible you need more power and/or a tank/atomizer that makes more vapor and/or draws easier. Free draw was a big issue with me: is whatever you're using hard to pull on, or as easy to hit as a cigarette? Unfortunately, the right combination of equipment and juice is so subjective you are the only person who will ever know when you get it exactly right... and "exactly right" is likely to change over time.
Posting us a pic of your tank, or a good description of the model and ohm rating of the coil, would help us advise you, if you can. Remember you've got friends here, who will try very hard to help you. We're in your corner and we want you to succeed. Vape on!