Context: A and B were involved in a sparring session, and after winning B exclaims they will cherish this memory forever. A responds:
The first weird bit is this (Which is a strange thing to command someone to do):
まあ、さっさと忘れろ = Now now, forget this quickly.
The later bit is colloquial, but not that heavy, and can be expanded back into:
The first bit にゃ=には, I'm not 100% sure. In any case the bit up to the comma is "From now on you"
The 覚えていかなければならない事 bit is my stumbling block. The verb itself probably means "have/should/must continue to remember" The interpretation would depend on what that 事 is doing here. Is it nominalizer? If so the sentence is strange: "Being able to having to continue remembering many." I think it's probably here as a noun, meaning an event or something to be memorized. So it's "things/events you have to continue memorizing". Have due to なければならない and continue due to ていく.
Mating that with 山ほどできるのだし。is still weird though. の is there to nominalize the previous bit, だ is there to make it a statement and し is likely there to denote this as one of reasons for forgetting this fight.
But still "From now on, you will be able to have to continue memorizing many things." I'm assuming we are supposed to figure out that できる refers to the ability to memorize, all these things that can't be forgotten?
So the final statement would be:
A: Now now, better forget this quickly. (If you do so)? from now on, you'll be able to memorize many more events/things that you must not forget.
But I don't think I really got the nuance of this one. In particular,
Is にゃ ねば or には?
What is the meaning of The 覚えていかなければならない事
And how does the previous fit with 山ほどできる