Find a suitable place to plant it - but only if you have a very large garden. These trees get very large indeed, if they do not succumb to any of the 3 diseases/pests currently afflicting them.

You could consider "bonsaing" it then keep it at about the size it is now if that size suits you!

Is there a place locally that you could plant it? I live near a park and woodland where I put trees that are too big. The kids like gathering seeds and seeing them sprout. 6 ft is a bit much. The problem is you are not going to be able to keep it happy in a pot for much longer and its need for water will outweigh what you can add and feed it, even if it is sentimental. I think it is too late to 'bonsai' as ideally you do that from year 1-2 and it sounds like it is tall and thin and not small and bushy.

The woodland trust are planting up jubilee woods all overthe country maybe they would like your tree,
they do grow quickly when planted out, the one I brought here 20 years ago was as tall as me.....now I can,t even hug it!

any tree is realy going to be happiest out of a pot and so are most plants as there a human invention that some plants deal with better than others .i like pamg`s idea as conkers thow not good for us are good munchies for deer,squirrels etc .