The correct adjective from 'stress' is 'stressful'. Just as something which causes you pain is 'painful', something which causes you stress is 'stressful'. Long-distance travelling is stressful. I had a stressful day at work today. 'Stressing' is the present participle of the verb. It isn't normally used as an adjective. I checked and found a few instances of 'stressing' as an adjective on the internet, but this is generally seen as incorrect. The majority of English speakers would not say this. These non-standard forms often occur in bilingual communities, in the US in particular. For example, Italian Americans might say 'I had a stressing day' as a direct translation of 'una giornata stressante', and the usage spreads from there. Don't imitate this. If you wrote 'I had a stressing day' in an exam it would probably be marked wrong. You should stick with the standard word 'stressful'.

"Stressful" is an adjective, and "stressing" is the present participle of a transitive verb, i.e., it needs an object. Thus you can say any of the following: "This exam is stressful", "This exam is stressing me", "This exam is stressing the use of English in Polynesia" or "This exam is stressful for me". "Stressed" is a past form of the verb "stress" which can also be used as an adjective meaning "under stress": e.g. "He stressed the use of English in Polynesia", "The heavy truck stressed the bridge", "The bridge was stressed by the heavy truck" or "I was stressed by the exam".