I think it is natural given all that you're doing and going through that there is a bit of a down phase in the relationship. That is not to say it cannot come back up again.

I would talk to him about the expectations in the relationship & about who you are. You don't have to pretend to be someone you're not. I, too, go through exercise phases and would want my husband to be to know that I cannot always be "on".

I don't think you really have anything to worry about though. He loves you, he reassured you that he wants to marry you and seems very committed. But a conversation about all this would be healthy to clear the air so to speak and to set realistic expectations for the relationship.

You certainly don't have to perfect... and every relationship will experience life difficulties that must be managed somehow --- but it's ok if there's a down phase, which is totally understandable. Just know that when things go down, inevitably they will come back up again, the sun will shine again and the sky will clear.

Perhaps make a romantic date night too during all of this for just the two of you.

HUGS.