Answer - There is a saying that the gifts of the spirit are recognised by their fruits. Broadly put this means that if something is good for you, the will know by the goodness it manifests in your life. If we examine your creature, by your own suggestion this creature (spirit, entitiy, ancestor, whatever) has been a source of support and strength to you. It has been present in the happier times in your life, absent when perhaps you were less needing of its help and reappearing when you hit rock bottom. I would say that (its less than pleasant appearance aside), this in a benevolent entity who wishes you help and not harm.
Your question ...what is it. Well I could give you over 100 possibilities and come no closer to the truth. It could be an encounter with a soul mate, someone you have met in the world before and who has crossed the barrier to support you in this life. It could equally be a projection of your own sub conscious. Many people find it easier to understand parts of themselves if they project them into being an individual in their own right. This could be the case, I am not discounting it. It is possible that your own dissatisfaction with elements of your life have evolved to create a representation of your fears who advises you to move forward/back off when you hit a rocky period in your life.
Having said that, it could just as easily be a benign spirit, akin perhaps to the Christian construct of the guardian angel. It could be equally be a projection of one of your ancestors, reaching to you from between the worlds to guide you forward through difficult times.
I do feel obliged to offer the warning that your creature may also be a low level spirit messing with your mind, but to return to my first paragraph, I think you would have been unlikely to see such positive results from your interaction. The fact also that you feel sadness at the absence of the creature would suggest the relationship to be genuine. I would also be inclined to discount the idea of a malicious little low level entity given the duration of your time with the creature. Most low level spirits have neither the intelligence nor the stamina to maintain any kind of relationship for any length of time.
You ask if it brought you happiness until you took it for granted. I can only offer an opinion but actually, I think not. I think this creature appears to you when you need it, not simply when you desire to see it. It is unlikely it is offended that you took it for granted. What I think is more likely is either a) the crossing of the worlds requires a good deal of effort on the part of the creature and it only expends the energy when you are in need or b) you are only receptive to the creature when subject to emotional turmoil. This latter explanation is not unlikely, when our primary senses fail us through whatever means, the spiritual senses will often step in to lend a hand. Also, desperation can lower your natural inhibition toward belief of other world creatures. When life is all going well, most people are perfectly happy to leave their knowledge of the other world to the books of JRR Tolkein. When the chips are down we are receptive to anything, anything at all that may offer a touch of comfort.
Do I think you should try to contact this creature. Yes, but I don't think it is as simple as whipping out your ouija board. On some level I suspect the creature may well be with you all the time, just generating the additional energy to become visible when you need the extra source of support. In which case, I do not think it is likely to put the effort in because you are curious to know more about it. You may feel it is absent but I wonder how much of that is about the creature and how much is about you. You talk of having an affinity with spirits in your youth and yes, this is a gift especially common in children. But have you allowed yourself to become too dragged down into the mundane. Is it a case of the creature not showing itself to you or is it a case that you are no longer able to allow yourself to see the creature. Have you let the problems in your life drag you down to such an extent that your own gifts have become jaded. And if so, the answer lies in enhancing your own receptivity as a person, not calling out to the creature directly.
I would finally offer the possibility that the reason you no longer see your creature is because its work may well be done. People, spirits, soulmates move through our lives and we gain succour from them. But sometimes as we move forward their help is no longer needed. This does not devalue the relationship, merely that we have learned all we can from those who have touched our lives. You have experienced a difficult divorce, career issues, poverty. And yet you have survived. If how to survive is the lesson this creature wanted to teach you, you have learned it well. Perhaps that was its intention. To help you over those immense hurdles as you move into a happier phase of your life.