Sounds to me as if you are attempting too low a cut at any one time. Try raising the deck by a wheel height adjustment, and then mow it several times, instead of trying to do it all at once. Did you sharpen the blade? And, allow the grass to dry. Wet grass will bog down the mower! Mow more often. This will allow the engine to run at higher RPM's, which should make it mow better. Don't let the grass get so high!

It was just so high because first time I mowed since spring, and its growing fast. I normally cut it at 3", and mow when it gets around 4". ( I read that is one of the better ways to cut it including only taken off one inch so you are not cutting more than 1/3 of that grass stalk) Also I always try to cut dry grass and constantly keep undercarriage and mower clean. When it was high when I cut it and cut it at highest setting and it was fine, then I cut it from like 5" to 3" and it bogged a lot, and even when lawn is 4" it still bogs and doesn't mulch the best. Can't I just get a more powerful motor that will fit and a better blade?

You can make such a change, but the mower is not worth the expenditure. I'd suggest a new mower with a 190cc or larger engine and a max cutting height of 4"+. When you have 5" of grass and leave the mower cutting height set on 3", you should expect exactly what happened. When you have to cut tall grass, sometimes you have to discharge the clippings and come back and bag them up later. Then the next time you can mulch if you can cut 1" or less. I think you probably need mowing lessons more than a new mower, but you should maybe take it by the shop to make sure its running right and at the max speed.