There aren't that many uses of those terms, one each I think. Hitting below the belt refers to boxing, which means it is a punch that is against the rules of the sport. Similarly, if someone does something that is rather mean-spirited or cruel in a friendly competition/argument/debate, then you could say "that was below the belt", or more commonly, that was a "low blow". The other one "under my belt" means something you have achieved, like passing a test, or learning a skill. You might say after passing a driving test that "Now that I have that under my belt, I can get back to studying English"

The expression "below the belt" is more like 不正当的 or 不光明正大的 or 玩弄卑鄙手段。The expression "under my belt" means to experience something successfully（被自己经历过）。For example, that remark was really below the belt. Or, I was not looking forward to that trip, and I am glad to have it under my belt. As for anger, consider the following few in order of degree: 1) disturbed 2) irritated 3) miffed 4) perturbed 5) upset 6) exasperated 7) cross 8) sore 9) piqued 10) ticked off 11) irate 12) aggravated 13) indignant 14) incensed 15) infuriated 16) enraged 17) furious 18) mad 19) choleric 20) postal

below the belt= inappropriate or offensive under my belt= an accomplishment