It can be difficult to move through the grieving process and back into life again when a spouse of many decades has passed away. The person left behind has only their memories of love and happiness to sustain them through the grieving process. Once that is over, they will need to view any new person they have a relationship with as someone separate from their past. Avoiding relationship comparisons is important if a person truly wants to move forward with someone new.
A faithful partner for many years is someone who should be cherished after they are gone, but they should never be the only measure of a good partner. Nominating them for sainthood after death is fairly common at first, but those memories should be tempered by reality. No spouse is perfect, and they should not be used to hold a new partner up to an impossible standard. While there is no issue with remembering the good times, there is one with constant comparisons.
Each relationship between two people will be different. A spouse of many years may have known everything, but their memory should not be used to force a new spouse to act in the same way. Being with someone new means there is a learning curve for both people. Old memories that have been wreathed in sorrow can interfere with the process of being happy with a new person. It can turn any potential relationship into one that will not be successful for either person.
There is nothing wrong with mentioning a former spouse who has passed, but they should mostly be kept out of any new relationship. Leaving them behind is not forgetting them, and it should be regarded as honouring them. A widowed spouse should be able to move on and find new happiness with another person even when it means learning to stop wishing for what they can never again have in a relationship.