Hi, I had a startup fail after we (I had 1 co-founder) invested ~4 years into it. Some thoughts:
A startup failing is not a rational thing. At this time emotions (regret, fear, stress, depression) take over, it's a tough time to handle. If it's tough for you, take comfort in knowing that it's tough for everyone. When I was going through this, I couldn't handle the stress, so it spilled over into my personal life, and in the end I also got divorced a few months after the startup failed. Avoid that by not prolonging the process.
Talking to too many people isn't that helpful. Most "naive" people (who haven't done this, who don't understand how tech companies work, etc) will just tell you to push on. They will do so simply because it's baked into our Culture, that when somebody is in trouble you should be positive to them, tell them not to quit, the way you would fire up a runner at a Marathon race. But a startup is NOT a Marathon race, because training for and finishing a Marathon is purely up to you, but that's simply not true for a startup. (As a Marathon runner, this was one of my misconceptions going into my startup.) Here on HN we're okay with failure, but outside this bubble it's not that okay. So if you talk to people (friends, relatives) you'll get roughly the same if you told them about "I'm dating a girl and it's not working out that great" or "I have a new job but it's not working out that great", most advice would be pretty useless and just along the lines of Try Harder. So you have to learn to ignore these non-expert opinions. The fact that they may be investors sucks.
It sounds like you really don't believe it's going to work out. Don't spend another 3-6 months proving to whoever's left (clients/investors) that it doesn't work, just so they "let you" off the hook. As somebody else said, go on a vacation, and if you still want to stop it, stop it.
One final thing: when I was going through this, I was constantly projecting bad outcomes, which in this first world scenario is a few tough/uncomfortable conversations/situations with people. In the end, some of these happened, but overall it was maybe 1-2 hours worth of conversations, which really weren't that tough/uncomfortable in the end. In retrospect, the fact that I stressed over it 24/7 for months (so, 100s of hours) was really stupid and irrational, so don't do that.
One final trick: imagine yourself 3 years down the line. EVERYTHING WILL BE ALL RIGHT.
Good luck :)