No, that makes a lot of sense and accords with most people's intuitive understanding of the word. The trouble is, as you say:
If you are wholly unwilling to enter into agreements with other people then you will find it very difficult to achieve anything at all
And economic 'independence' is a real problem if you want to have a non-agrarian society. You need to import oil, electronics, pharmaceuticals. This is what goes wrong in places like Venezuela and Greece when supplies of hard currency start running low.
What exactly is your definition of sovereignty?
Am I correct in assuming you are a supporter of the TTIP because you belive it will somehow increase the sovereignty of the EU?
pjc50's sibling comment says that maximising sovereignty by declaring one's personal independence accords with most people's intuitive understanding of the word, but I really think that can only be at the most basic level of analysis - a moment's additional thought leads you to how you interact with others, and whether they might, for example (and to be not unhistorically hyperbolic) enslave you.
Sovereignty is about being able to make decisions, and in practice (which is all that really matters) that requires deciding which other people you are going to interact with, and how, in order to get along in the world.
As to your final question, I don't have a good answer. My instinct is to be against TTIP, largely because of the people negotiating it - I imagine it will increase their sovereignty quite a lot, but probably not mine ;-).
My friend, 52% of Britain agreed with you and voted to leave the EU !!!
"Freedom" to act is not a state of nature. Others can inhibit your freedom unless you convince them not to, either through agreement or force.
If you don't think that the EU is a power-sharing agreement that the UK has now voted to withdraw from, what do you think it is?