Is it easy to find a partner and/or maintain relationship when you have Porphyria?

Yes! as easy or as difficult for anyone else! you have a mutation or disease, you are not disabled, you just have a disease

It shouldn't be anymore difficult to have a relationship or find a partner than any other medical condition. The hard part for a partner is to understand what the person is going through, on the outside one may look healthy but on the inside its another story.
Everyone suffers different levels of pain, daily symptoms can be difficult to deal with, outdoor activities may be severely restricted if you have one of the cutaneous types and this can often be difficult to deal with. Mood swings and depression can be hard on a partner but this depends on the person and the support / management structures in place.
If you are in a relationship with someone and they really care for you this should not be a drawback. People who are confined to a wheelchair, or those who have motor neurone disease are able to have relationships therefore having Porphyria should not be any different.
You may have some lifestyle limitations and may need to make some adjustments however there is nothing that cannot be overcome. Honesty is the best policy when meeting someone new, slowly introduce them to your world, don't overwhelm them all at once with information because its a considerable amount to take in. Discuss the implications with them and let them read up on the disease. Let them know what to do in the event you have an attack so that they are prepared and know what to expect.
It may not always be easy on a partner especially in times where they may see you experience psychotic episodes or deal with your anxiety or depression issues, however these things can be overcome and managed if there is support in place for the family / partners of Porphyrics.