When you say "I can't but X", you mean either (1) that there is no other option or (2) that it is beyond your control or (3) that doing something else is not helpful.
(Eg 1) "I lost my wallet, so I have no money for taxis or buses. But I have to be in office soon. I can't but walk" , "I do not know cooking. I can't but eat out" , meaning : no other option for you.
(Eg 2) "Jokes by Chris Morris are always hilarious. I can't but laugh whenever I hear them" , "Wine is addictive. I can't but swig a bottle every night" , meaning : it is beyond your control to not laugh or not swig.
(Eg 3) "Instead of buying cake for $10, I thought I would save money by making it at home. I ended up spending $40. I can't but laugh at myself" , "I lost a lot of money in gambling. I can't but blame myself" , meaning : crying or cursing is not helpful.
Meaning (2) is related to "I can't stop laughing" & "I can't help laughing" because "I do not have control".