Buyers remorse is not at all uncommon. Rest assured that when you move onto moving day you will remember how much you loved the home and why you moved forward with your offer. There is value to the life that you will be enjoying in your new home.
For future reference, there is great value in working with a buyer broker who can help you to establish and understand value. Keep it in mind for your next transaction.
First of all, breaking a contract can cost you your deposit money. You must read the contract and see what right both the buyer and seller have in a broken contract if you are thinking of backing out. If there is a large deposit involved you should consult a real estate attorney.
Secondly, if you like the house and don't want to break the contract, the mortgage commitment could be your last line of defense. If you do not receive your mortgage because your property did not appraise, most likely you have a mortgage contingency clause that can protect you. As Emelia pointed out the banks interest mirrors yours and if it can;t appraise they will not lend.
You do not need to worry about paying too much your lender will not let you. When the appraiser comes out to value the property they will tell your lender what they think it is worth and will only lend you that amount. Why when you are making probably the biggest purchase of your lifetime you have no guidance?
The first rule of buying is to assess what the value of the property is in todays market and base your offer on that. If you already made an offer, it may be too late to bring in another agent but you may want an attorney to review what you have signed. If you already entered into an agrement you would need to use a contingincy to get out, at this point it is unlikely you can renegotiate a price if you already agreed to it.
good luck working things out