I believe that psychological readiness for a serious relationship can be determined by the level of flexibility which a partner is ready to show towards the supports holding his or her dignity, self-respect and pride.
Probably, readiness for new relations depends on how well one has managed to cure dignity wounds left by previous relations, how well his or her ability to get deceived has restored. New relations mean new illusions.
As for a serious relationship (when a partner is chosen for a long term which is felt like forever) readiness for relations is determined here by the presence of previous experience. If there have been none there is no readiness.
That is why the experience of love and consistency in the family is extremely important for a young person without an experience of serious relations.
For a long time vs with dignity
Psychological readiness for serious relations is determined by the scale of their mental map, i.e. whether you subconsciously consider them long-term or get ready for a quick split-up.