IMA, I am sorry you are in such a horrible, painful situation.
It seems to me that the heart of the problem is that you have absolutely no control over what your partner chooses to do. None of us has that control, of course, which is why we need the trust you talked about in order to feel secure. Trust is terribly difficult to rebuild when once it has been shaken, much more difficult than to trust someone who we know less well, but who has never (yet!) shaken our trust. But that re-building is not a one-way street. We can't just decide one day to trust again, regardless of what our partner is actually doing: that would just be wilful self-delusion. We need to see evidence that s/he truly means to do better and is therefore worthy of that trust again. So we have a situation in the balance: it could go either way, and we will not know which way until we have seen whether s/he will actually do it again or not. The only thing we can do, if we have decided to try and make the relationship work, is to let go and see what s/he does with the freedom. If s/he does it again, then we need to re-think our options: is it actually more painful to be in a relationship with this person, or to be without them? There is no right answer to this. It is not a matter of what we *should* do, what other people or our culture tell us is right for us to do or feel, but what is actually the least painful option in a horrible situation.
I understand what you mean when you say you worry about the behaviours that were pre-cursors to problems in the past: you are sensitised to them and even if they are absolutely innocent this time the anxiety is there that they might not be; they have become a fear trigger. If I was in your situation, having decided to try to make it work again, I would try to keep an open mind about these behaviours, difficult as it would be. I would express my anxiety about them, but only once so that he could understand why I was anxious and would have the opportunity to take my feelings about them into consideration. What he decided to do about them then might be a powerful message about how much he cares about my feelings. For example, if I was worried about him sending texts, he might (for now, at least) offer to give me open access to his phone and not delete anything till I had read it, to help me feel more secure about it. On the other hand, mentioning my anxieties about these behaviours repeatedly could lead to pointless, painful arguments and probably make him feel angry about being mistrusted. Not that he would have any *right* to feel that way, but people who are really trying want to feel trusted and their effort appreciated, so mistrust may push them back to old behaviours on the grounds that they are not trusted anyway, so they might as well do as they please. And people who are *not* trying get angry and defensive because they know they are in the wrong, so they try to turn the discussion around and put you in the wrong instead. Either way, it is pointless and painful for the injured party.
I am very much afraid that this looks to me like a 'wait and see' situation, not a situation that you can improve by simply deciding to trust again in the absence of any real sign of improvement. You have real reason for the mistrust you currently feel, and it may take years before your full trust is re-established.
But believe that it is possible for a person to change radically, to turn their life around, if they are determined to do it, and if they are prepared to work at it and keep working at it even when it is hard. It is difficult for a person to alter an entrenched bad behaviour pattern, but easier to do when they have someone who believes in their ability to do it, who *expects* them to keep trying, and who trusts their good intentions, if not their inclinations.
My thoughts are with you.