gosh Mike that does sound like your having a terrible time of it at the moment, i really feel for you,you must feel so helpless with everything that's going on.
i think the feelings of "is that it" etc. are feelings that surely must be entirely natural at such a frightening time, you are questioning life itself, the meaning of it.
Im reading a biography of Tolsoy by a.n. Wilson, (sounds hard going but its very interesting and picked it up for 50p from the book farm) and altho he has just written war and peace, he is married, secure, wealthy, he is driving himself mad with worries of "is this it" everything he does, he thinks is pointless,as we are all going to die at the end of it all,he constantly worries that he will kill himself,even writing about it in anna karenin.
so even the biggest writers have the same worries as you have today! surely a sign of an intelligent person such as yourself.
whether in good health or bad. i guess what im trying to say, is it must be normal for these feelings,and they will not simply go away, you must learn what will work for you,and recognise they are thoughts, but intrusive ones.
counselling could help, and im sure being in your garden will bring some calm.
i wish you well through this difficult time x