I did not have valve repair, but did have an infected MV, and a strained MV.
It could be you need a nice gradual rehab program that minimally strains the heart as it strengthens it. You could be overdoing it, so you stay in oxygen debt.
I think the key to my rehabs, was that I used a monitor, and when the heart rate started to go up with no increase in speed or incline, that meant fatigue was setting in even though I could not feel it. I then did my 5-10 minute cooldown routine, primarily walking slower and slower.
I started these at only 1.5 mph, and went up 0.2 - 0.3 mph every three minutes if my heart rate stayed within 3-5 beats with the increase and I did not breathe any harder. It took awhile to get so I could go 20 minutes. But everything improved slowly. I later had an emergency triple bypass, but also developed excellent full collaterization of my heart from decades of exercise and, I think, my careful rehabs (4) in prior years.
Always take 10 minutes of slow warm-up prior to exercise. It helps oxygenate the heart, and makes exercise much easier.