On top of that, I'm gaining appreciation, debt paydown and tax benefits. 

If I understand it correctly when there is no cash flow or negative cash flow, it probably makes sense to sell as there will be tax benefit on the Capital gains vs Cash flowing property? 

In this instance, I would recommend finding local flippers or wholesalers in your area to teach you the ropes and you become their apprentice thus gaining experience in the field and an income.Whatever you choose to do, remember that we are currently in a seller's market where prices dwarf 2008 prices in many areas and so getting a loan for a property as a 17-18 year old can be next to impossible at a favorable rate.

You can deduct your mortgage interest, defer your capital gains indefinitely or even qualify for an exemption on capital gains altogether.

Take the tax free gain and buy one or two new properties with your profits/equity.

Your loan balance/equity has no bearing on capital gain (profit).  

Most appreciation is % based so it’s just s likely you gain more appreciation in the future than the past.

It is always better to ensure you have equity gain, rather than just getting lucky that the market went up.

How to get keys to vendors to gain access?

It added $90 to his existing mortgage, but he gained $300 cash flow from the new property, so it worked out.