Debbie,
If you are filled on your bid, then immediately place an offer on the other side of the spread, there is a remote possibility that if you bought at the very top of the market, prices will move against you and your offer will remain unfilled.
However, futures markets have "Designated Market makers" who are obliged to publish prices they are willing to buy and sell at for circa 90% of the trading day. If price moves against you, it will always be possible to close "at market" and hit whatever price is available from the MM's - this might be a bit off your entry, but you should be able to get out immediately.
The ES contract is very liquid; as I write, there are 11 @ 54.25; 54.50 for 73 - remember this is out of hours too. It is wholly unlikely that as soon as your bid or offer is filled that everyone leaves the market, there will always be a price to close at. I assume you will only be trading 1 contract to begin with, which shouldn't pose any problems with depth.
Of course, if you Buy at highs and sell at lows, a loss will be inevitable - but in either profit or loss, for 1 contract, you will always be able to get out - maybe at the wrong side of the spread, but during market hours in the ES front month, it'll only be about a tick off anyway.
I suggest you try it, placing an offer and trading out at market immediately afer your filled, with one contract. It should cost you 2 ticks plus comissions if you are very unlucky, call it $30, so write that off just to see that it can be done (call it $100, for the trade and the bottle of wine you can buy to celebrate 5 years of study into practice!).
Good luck!