Clearing the infield - firstly you don't always need to, even with a full ring field you're still far more likely to hit a gap than you are a fielder, so long as it's well hit you've a decent chance of getting it through. For when you do decide to go over, don't try and over hit it - if they're on the circle you've only got to get it 30ish yards to go over their head so a clean connection is far more important than power. In nets I'd try and practice staying still, keeping your head down and hitting cleanly through the ball, and build your confidence up before you try it in games. The usual big mistake is trying to hit it too hard and making poor contact, if you just learn to trust yourself and time it then you'll find it far easier. If you can find a willing mate to give you some throwdowns to practice giving it a smack then that'll be very helpful. The key to hitting a long ball is staying still and timing it, if it's in the middle of the bat it'll get over the infield the vast majority of the time.
Balls on the stumps - remember to respect the good balls when you need to! One thing as you're a leftie - from right arm over, unless it's swinging back into you then most balls on the stumps will be pitching outside leg, so make sure you get your pads in the way and it's a free hit. Otherwise, pick your shots on length - if it's too full or short then it should still be a low risk shot for you on a good pitch, respect the ball when it's on a good length. If they're always on a good length then well done to them, do your best to put them off it! Personally, if a seamer is hitting a length consistently I usually go down the track looking for a single - low risk if you cover your stumps as you're unlikely to be given out lbw if you miss it, and you don't have to do it much to put a bowler off their length.
Spinners - if they're on a good length all the time then you'll have to do something about it if you want to play it well, and that's footwork or sweeping. Right forward or right back to make the length fuller or shorter, or use the sweep to hit off length if you play that well, but pick your ball to do it - don't just run down the track to everything or sweep wildly. 99% of spinners will have men on the boundary somewhere, so look to hit the resulting gaps and run your singles/twos hard, and the poorer balls will always come. Don't be afraid to take a sensible risk if you need to put them off their length, going right back on your stumps or down the track (even if you then just block it) will give them more to think about. Try to make yourself unpleasant to bowl to - there's nothing better for a spinner than a batsman who just stands there and plods it back, if you can keep rotating the strike by just safely hitting gaps then they'll find bowling to you far less comfortable. For practice - concentrate on making length for yourself when facing spinners in the nets, either getting to the pitch of the ball or going right back to make it short. Once you're happy doing that then you should be creating scoring opportunities much more easily.
Bit of a splurge but hopefully that's helpful!