I'm not sure I have an absolute "definition" of sovereignty, but I think it is approximately a measure of how much power an individual or group have over their own affairs. A failure to recognise that power can be restricted by external agents is simply a failure to confront reality.

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 ;-).

Exactly sovereignty is being able to make a decision. When those decisions are made for you, you lose sovereignty.

My friend, 52% of Britain agreed with you and voted to leave the EU !!!