I say hold on to the jacket. I remember myself when I was twenty, an eyeblink and a decade later and I'm 40lbs heavier (I hope you fare better with age). You may find you will grow into it, both physically and sartorially. I don't think its as terribile as the other posts seem to be painting it.
The pictures are not clear, but before taking any drastic advice, try posting again with higher res pics after a good pressing. We need a better sense of the fabric, is it the typically stiff scratchy tough Harris Tweed, or is it on the softer, stretched out side? Wrinkles aside, Its a bit roomy at the chest and wide at the shoulders, but your are not quite swimming it it. A light sweater underneath may help, buttoning the top button may also clean up the top.. You look like you could pull off a 3 button, but may want to see how nicely the lapels will roll to 2.5 (the lower lapel edge doesn't look too flat, but the defined crease underneath the top button is not encouraging on that front). Ironing and shaping will reveal what those lapels are capable of (make sure whomever does this for you is competent). The sleeves seem sort of wide in profile, but are not objectionably large when viewed from the front, in fact the upper arm is free from many of the creasing/twisting that you read people complaining about.
The bottom "bell-shape" (pear shape) is something you can ask a tailor to fix right now. The lower perspective of the 1st picture makes it look worse, but it is too wide, even if you like open quarters. I've had this done on many jackets, it hasn't been a problem for my tailor.
Back balance is a little long, the original owner's posture probably wasn't as erect as yours. If you naturally stand this straight and your shoulders back, I'm not sure if anything can be done. But if your shoulders and head fall a bit forward during normal life, it will look better.
Hmmm, seems like my underlying advice is gain weight and have worse posture.
