Hi inthe,
Okay, so here's some things. Well done you for being strong and pro-active. I love how you say you are not going to be anyone's fool.
The things I'm about to say right now do NOT necessarily apply to you. I'm going to speak from a place of personal experience, statistics and experience of these boards.
It is extremely rare for a partner, particularly a man, to come to a relationship with an 'I'm not happy / I love you but am not in love with you' without an explanation and that be the whole truth. Adults have explanations for things. However, if there has been a lengthy period of mutual relationship dissatisfaction that has been discussed, then it is pretty common and often instigates a break up. If this is not the case it is almost always about some form of infidelity.
It is also very unusual for anyone who has framed the situation in this way to admit to infidelity. Most cheaters deny and all minimise.
I think it is extremely likely that your partner has been unfaithful in some way. I'm sorry. Even infatuations tend to be kept quiet about, to get him to say he is unhappy means the OW is likely to have expressed an interest too. So whether it is an emotional or physical infidelity, it is likely something is going on. your instinct as to who it is is probably right, though not definitive. Sometimes cheaters confide in someone of the opposite sex they are close to.
What is cowardly and sad about your husband's behaviour is that he is now putting all the onus on you to act. This is common for cheaters. They want to test the waters in the primary relationship to see if their partner will fight for them and do the 'pick me' dance, or leave them to be free to see the lover. but they don't want their hands to be seen as dirty while they test this. What follows is almost always a worse midway - a long, painful period of hedging and uncertainty and far far more pain than is necessary. The reason many advise to snoop at the point is because the truth needs to be brought out into the open and your husband's denials are just self-protective flailing. The fact he says he doesn't like himself at the moment is a symptom of guilt.
Unfortunately, and through no fault of your own, you are now in a terrible position and it will put a strain on your mental health as your husband likely gaslights you through the next little while. The best case scenario is that he, destabilised by your response, drops contact with the OW (or crush) completely and focuses on repairing the damage he has done to your trust. This is possible but unlikely.
If you cannot find proof, your choices are now, contact the OW you suspect (most people don't want to do this because of their pride and belief that she would lie anyway).
Tell him to leave the house if he is uncertain about your marriage and to only return when he is absolutely sure and with the whole truth, when you will decide what you want.
Or go into counselling in the hope that the truth will be forced out of him there.
Most people who act go for 3 which may be a good bet in your case. Since you pushed the confession of unhappiness out of him, there's a slim chance he might just be a bit depressed and disappointed in life. Most people do not choose option 2 because of the upheaval and the risk of losing their partner. This is totallly understandable. I can only say that almost every betrayed partner I know wishes they HAD done that at the point of the 'i'm not happy' speech.
What's upsetting is this could be anything from a long term full blown affair to a kiss at a christmas party, to a mild flirtation. Without knowing your husband, who knows. Also it is worth saying that you believe him when he says there isn't anyone else. Instincts are good. What worries me is that your descriptions of your husband are a bit damning. his 'wobble' at 30 does not speak well of him. Neither does your description of slightly drifting through the next ten years. He seems quite passive and this is exactly the kind of person who lies about infidelity and then wants someone else to make their mind up about what comes next.
I am sorry you are going through this. I am aware people will think I'm being hasty and presumptuous. To them I can only say I am glad you think that. That speaks well of you and your relationship.