It sounds like you definitely have a claim for adverse possession. You would need to file a "Quiet Title" action in the Land Court (Circuit Court). It would be wise to get a real estate attorney to file it on your behalf.
In Massachusetts you must possess the property being claimed, continuously for 20 years (Mass. Gen. L. ch. 260, sec.21) To meet the requirements for adverse possession you must also show that:
1) You were the exclusive possessor and actually entered the property.
2) Your possession must be open and notorious--your possession must be seen. The possession must be appropriate to the type, size and use of the land. Enclosures, houses, cabins, payment of taxes all help establish your claim. The general idea is to give the owner reasonable notice that you are in possession and give him the opportunity to eject you.
3) Your possession must be adverse to the owners claim, in other words without the owners consent. If the owner has given permission for you to be on the property you can't claim the property adversely.
4) Your possession must be continuous (for 20 years). If your entry was only occasional you may be deemed a trespasser and not be able to claim adverse possession. However, certain seasonal or intermittent uses satisfy the continuous element if the average owner of a particular piece of property would use it in that manner (e.g. a summer home).
Massachusetts has a special land registration system in which a question of title can be brought to the Land Court which investigates and evaluates the merits of the claim. Based on its findings the court can issue a new certificate of title. A further advantage of this procedure (to the owner) is that property registered through the land court cannot be adversely possessed.
It looks like you meet all the requirements and it shouldn't be too difficult to establish your claim. Time is your enemy so you need to move on this quickly before the neighbor starts an action that will cost you money to defend against.
I hope this helps.
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