It is perfectly reasonable to want time apart.

I'm not sure what would convince him of that, using the word "no" is a standard recommended assertiveness technique, amazingly powerful in my experience, then explain you care about him just as much and explain why you need time..if it doesn't work repeating it might help....

you'd have to judge for yourself if this is wise with this particular person and situation.

I saw on some TV documentary it's good for couples to spend time doing things apart so they have things to talk about, these relationships last longer.So even when things are fine time apart is a good idea.

My only reservation is that if you are bipolar there's a chance you want to be away from people because you are becoming depressed, you and those around you might want to keep an eye on your mood, perhaps mention it to your doctor next time. On it's own it doesn't mean anything but might be one symptom.

Good luck