By waiting so soon before the primary he made his statement. Leave it at that. He is hoping to make nice with the GOP as they have accepted him and since Ryan endorsed him back in June he’s playing nice. Once Ryan loses he can endorse the other guy.
It appears you may be correct. That might be relevant if this were simply an ego issue; however, there is an important political issue at stake.
There is a real risk that Ryan is owned by powerful/influential folks whose objectives are much different than Trump's. With or without Trump's endorsement, a Speaker Ryan could well be an significant obstacle to the implementation of Trump's plans.
In any event, what is to be gained by endorsing Ryan? What could very well be at stake are undecided voters who question whether Trump is strong enough to be a political leader. What most of us respect is that Trump is an ass-kicker who it appears will answer to only to common sense and American "make my day" principles.
If you break your vow and take what you hope will be a fatal shot at the king, you should not expect that an apology will save you. If Trump's non-endorsement throws the election to Nehlen, it would be an economical way to solve two issues with one action.