As you had just stated, ignorance is not bliss. Deciding to ignore P to be happy, will only keep you happy if you don't think about P. The fact is, you know that P exists, and by ignoring P, you ignore what would make you upset. And what would make you upset, exists.
The fact is, P could be so many things, P could be a sickness. If you choose to ignore it, you will be happy doing your regular routine for awhile, but by ignoring it, you let the sickness get out of hand, forcing you out of your regular daily routine longer than if you had just accepted the fact that you had been sick.
P could be your spouse cheating on you. By ignoring this, you can act like everything is alright in the relationship, but you would be doing the same thing that made your spouse cheat in the first place. If you keep letting it happen, you will be left for someone new, or it will come to a point where the whole relationship/marriage can never be fixed.
P could be you getting fired in a few days. You can ignore it, by keeping on doing your daily workload. Thought that is probably the reason your company is letting you go. By accepting the truth, and working really hard, you may just keep the job, of course you will be miserable working incredibly hard, but you will be happy knowing you avoided losing a job, and what could have been financial problems.
You could be the doctor, P being the sickness of your patient. Would you choose to ignore it? You would be miserable telling your patient can die, but you will have given the chance to someone to try and save their own lives.
P could be so many things you wish weren't true and decide to ignore it.
By ignoring these things, you can act normally while these things manifest into something much bigger. That happiness you get from ignoring it, will be short lived.
Though in some cases, you could have already done something wrong, and by ignoring P, you could only be happy for awhile.
You could be the one cheating on your spouse, and P could be that she will be leaving you. You could be happy, temporarily, to be sleeping with someone else, but if you choose to accept that your spouse is leaving you, you will be miserable, but you will eventually fix the marriage.
You could be doing drugs, heard about a drug test coming up, would you ignore P, which would be that drug test? You could be miserable by staying away from drugs, but you will have avoided a huge problem by being clean in the drug test results.
To conclude, you can ignore problems, until they manifest into bigger things, and affect bigger areas of your life. And by realizing that P is true, you can avoid bigger problems in the road ahead. Though you will be happy ignoring it, you won't be happy for very long because emotions always change to the current situation, and the the situation always changes. Just like how you would be miserable knowing that P is true, you being miserable is just for awhile. So being in the state of acceptance that P is correct and being miserable, is better.
So with that being said, does it really matter what P is? No, P doesn't matter as long as believing in P will make you miserable, and ignoring it will make you happy. If P is significant enough to change your emotions, then what P is really doesn't matter since no matter what P is, it changes your current state of emotion.